| Debt (Details) (USD $) | 3 Months Ended | 9 Months Ended | 9 Months Ended | 9 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011 | Dec. 31, 2011 | Sep. 30, 2012
 Term loan [Member] | Dec. 31, 2011
 Term loan [Member] | Jul. 31, 2011
 Term loan [Member] | Sep. 30, 2012
 Letters of Credit [Member] | Dec. 31, 2011
 Letters of Credit [Member] | Sep. 30, 2012
 Revolving Loans [Member] | Dec. 31, 2011
 Revolving Loans [Member] | Sep. 30, 2011
 Revolving Loans [Member] | |
| Debt Instrument [Line Items] | |||||||||||||
| Debt Instrument, Maturity Date | Jun. 30, 2016 | ||||||||||||
| Financing costs | $ 1,700,000 | ||||||||||||
| Borrowings outstanding | 96,800,000 | 110,900,000 | |||||||||||
| Term loan payments, 2012 | 4,700,000 | ||||||||||||
| Term loan payments 2013 | 23,400,000 | ||||||||||||
| Term loan payments 2014 | 25,000,000 | ||||||||||||
| Term loan payments 2015 | 34,300,000 | ||||||||||||
| Term loan payments 2016 | 9,400,000 | ||||||||||||
| Revolving loans maximum borrowing capacity | 125,000,000 | ||||||||||||
| Revolving loans potential increment in maximum borrowing capacity | 175,000,000 | ||||||||||||
| Debt outstanding | 148,800,000 | 148,800,000 | 164,200,000 | ||||||||||
| Revolving loans amount outstanding | 52,000,000 | 53,300,000 | |||||||||||
| Letters of credit outstanding | 1,300,000 | 5,000,000 | |||||||||||
| Debt Instrument, Description of Variable Rate Basis | LIBOR base margin 0.50% | ||||||||||||
| Debt Instrument, Basis Spread on Variable Rate | 2.25% | 2.25% | |||||||||||
| Duration of interest rate cash flow hedge (in years) | 3 years | 2 years | |||||||||||
| Amount of interest rate hedge | 101,000,000 | 20,000,000 | 40,000,000 | 40,000,000 | |||||||||
| Effective interest rate on the hedged term debt | 0.56% plus base margin | 0.7775% plus base margin | |||||||||||
| Interest rate range, minimum (in hundredths) | 2.47% | ||||||||||||
| Interest rate range, maximum (in hundredths) | 3.87% | ||||||||||||
| Effective interest rate (in hundredths) | 3.33% | 3.33% | |||||||||||
| Interest expense, net | $ 1,400,000 | $ 1,300,000 | $ 3,900,000 | $ 1,900,000 | |||||||||
| X | ||||||||||
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- Definition The percentage points added to the reference rate to compute the variable rate on the debt instrument. No definition available. 
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| X | ||||||||||
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- Definition Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date before deducting unamortized discount or premiums (if any). May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. No definition available. 
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition When presenting a range of interest rates, the highest stated (contractual) rate for funds borrowed under the debt agreement as of the balance sheet date. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition When presenting a range of interest rates, the lowest stated (contractual) rate for funds borrowed under the debt agreement as of the balance sheet date. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
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- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees). No definition available. 
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- Definition The reference rate for the variable rate of the interest rate derivative, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. No definition available. 
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- Definition The cost of borrowed funds accounted for as interest that was charged against earnings during the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. No definition available. 
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- Definition Amount borrowed under the credit facility as of the balance sheet date. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Maximum period the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Aggregate notional amount of all interest rate derivatives designated as hedging instruments in cash flow hedges. Notional amount refers to the monetary amount specified in the interest rate derivative contract. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Potential increment in maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. No definition available. 
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