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Other Contingencies (Details)
6 Months Ended
Jun. 30, 2017
USD ($)
financial_covenant
Jul. 11, 2017
USD ($)
Jul. 03, 2017
USD ($)
Dec. 31, 2016
USD ($)
Short-term Debt [Line Items]        
Bank line of credit $ 200,000,000      
Borrowings under line of credit $ 16,182,000     $ 0
Number of financial covenants | financial_covenant 1      
Reduction of bank line of credit $ 70,700,000      
Amount available under line of credit 129,300,000      
Subsequent Event        
Short-term Debt [Line Items]        
Bank line of credit   $ 300,000,000    
Amount available under line of credit     $ 125,800,000  
Standby Letter Of Credit        
Short-term Debt [Line Items]        
Irrevocable standby letter of credit, outstanding $ 70,700,000      
Standby Letter Of Credit | Subsequent Event        
Short-term Debt [Line Items]        
Irrevocable standby letter of credit, outstanding     $ 74,200,000