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Long-Term Debt - Senior Notes (Details) - Senior notes
12 Months Ended
Dec. 31, 2017
USD ($)
tranch
Debt Instrument  
Debt instrument face amount $ 800,000,000.0
Number of tranches | tranch 2
Debt issuance costs $ 11,700,000
Senior Note Maturing December 2025  
Debt Instrument  
Debt instrument face amount $ 400,000,000.0
Stated interest rate 4.625%
Senior Note Maturing December 2027  
Debt Instrument  
Debt instrument face amount $ 400,000,000.0
Stated interest rate 4.875%