401(k) Plan (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
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| Defined Contribution Plan [Abstract] | |||
| Defined contribution plan expense | $ 6.2 | $ 6.3 | $ 4.8 |
| X | ||||||||||
- References No definition available.
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| X | ||||||||||
- Definition Amount of cost for defined contribution plan. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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