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Debt (Details Textual) - USD ($)
3 Months Ended
Oct. 14, 2015
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Oct. 04, 2013
Line of Credit Facility [Line Items]          
Payments on long-term debt   $ 15,000,000 $ 15,000,000    
Term Loan Facility          
Line of Credit Facility [Line Items]          
Debt Instrument, Face Amount         $ 400,000,000
Payments on long-term debt   15,000,000      
Revolving Credit Facility          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity         $ 1,000,000,000
Revolving Credit Facility   0   $ 0  
Line of Credit Facility, Current Borrowing Capacity   419,800,000   553,500,000  
Senior Credit Facility          
Line of Credit Facility [Line Items]          
Debt Instrument, Maturity Date Oct. 14, 2020        
Letters of credit outstanding   $ 89,600,000   $ 102,600,000  
Line of credit, commitment fee (as a percentage)   0.15%      
Credit Facilities Scheduled Repayments Due in Next Four Quarters   $ 15,000,000