Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies (Narrative) (Details)

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Commitments and Contingencies (Narrative) (Details)
$ in Millions
 1 Months Ended 12 Months Ended
Apr. 01, 2016
plaintiff
Nov. 30, 2016
USD ($)
defendant
plaintiff
Apr. 30, 2011
subcontractor
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
May 01, 2012
USD ($)
Loss Contingencies [Line Items]            
Capitalization of construction costs           $ 27.3
Operating leases, accumulated depreciation       $ 10.9 $ 9.0  
Lease liability included in long-term lease obligations       20.3 21.7  
Current portion of lease liability       $ 1.4 $ 1.3  
Number of deaths of subcontractor employees | subcontractor     5      
Operating Leases | Minimum            
Loss Contingencies [Line Items]            
Lease term       2 years    
Operating Leases | Maximum            
Loss Contingencies [Line Items]            
Lease term       15 years    
Building Facilities            
Loss Contingencies [Line Items]            
Lease term       15 years    
Aviation Litigation            
Loss Contingencies [Line Items]            
Number of plaintiffs | plaintiff   5        
Number of defendants | defendant   5        
Number of deaths | plaintiff 3          
Number of injured | plaintiff 6          
Damages sought   $ 1.0