Debt - Additional Information (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Interest expense for premium paid for early redemption of notes in excess of carrying value	$ 89,000,000		$ 183,000,000	$ 203,000,000	$ 298,000,000
5.72% Notes, due 2019
Debt Instrument [Line Items]
Long-term debt, interest rate	5.72%	5.72%		5.72%
Long-term debt						$ 500,000,000
Redemption amount		$ 518,000,000
Loss on prepayment of debt redemption		$ 19,000,000