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Other Contingencies (Details)
9 Months Ended
Sep. 30, 2017
USD ($)
financial_covenant
Oct. 02, 2017
USD ($)
Dec. 31, 2016
USD ($)
Short-term Debt [Line Items]      
Bank line of credit $ 300,000,000    
Borrowings under line of credit $ 25,000,000   $ 0
Number of financial covenants | financial_covenant 1    
Reduction of bank line of credit $ 74,200,000    
Amount available under line of credit 200,800,000    
Standby Letter Of Credit      
Short-term Debt [Line Items]      
Irrevocable standby letter of credit, outstanding $ 74,200,000    
Standby Letter Of Credit | Subsequent Event      
Short-term Debt [Line Items]      
Irrevocable standby letter of credit, outstanding   $ 77,600,000