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Fair Value of Financial Instrument (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of plan assets   $ 601,700,000
Derivative assets $ 7,800,000 6,500,000
Derivative liabilities 17,000,000 13,400,000
Long-term debt 324,700,000  
Debt, face amount 305,300,000  
5 7/8% Senior notes due 2021    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt, face amount 305,300,000.0  
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of plan assets   537,200,000
Derivative assets 0 0
Derivative liabilities 0 0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of plan assets   0
Derivative assets 7,800,000 6,500,000
Derivative liabilities 17,000,000 13,400,000
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of plan assets   23,800,000
Derivative assets 0 0
Derivative liabilities $ 0 $ 0