Long-Term Debt - Senior Notes (Details)	Dec. 31, 2017 USD ($) tranch
Debt Instrument
Debt instrument face amount	$ 800,000,000.0
Number of tranches (tranche) | tranch	2
Senior Note Maturing December 2025
Debt Instrument
Debt instrument face amount	$ 400,000,000.0
Debt instrument stated interest rate (percent)	4.63%
Senior Note Maturing December 2027
Debt Instrument
Debt instrument face amount	$ 400,000,000.0
Senior notes | Senior Note Maturing December 2027
Debt Instrument
Debt instrument stated interest rate (percent)	4.88%