Debt - Issuance of Senior Notes and Repurchases and Redemptions of Senior Notes (Details) - USD ($)	3 Months Ended		12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017
Debt Instrument [Line Items]
Debt issuance cost		$ 6,000,000	$ 11,000,000	$ 6,000,000
Premiums paid on early redemption of debt		30,000,000		30,000,000
Accrued interest on early redemption of debt		9,000,000		9,000,000
Gain (loss) on early extinguishment of debt	$ (22,000,000)	$ (32,000,000)	$ (22,000,000)	$ (37,000,000)	$ (19,000,000)
4.875% senior notes due 2027
Debt Instrument [Line Items]
Face amount of notes	500,000,000
Stated interest rate			4.875%			4.875%
Debt issuance cost	7,000,000
5.5% senior notes due 2025
Debt Instrument [Line Items]
Face amount of notes					$ 350,000,000
Stated interest rate			5.50%		5.50%
Debt issuance cost					$ 6,000,000
7.75% senior notes due 2018
Debt Instrument [Line Items]
Stated interest rate		7.75%	7.75%	7.75%
Repurchase amount			$ 500,000,000
Aggregate consideration			536,000,000
Premiums paid on early redemption of debt	20,000,000
Accrued interest on early redemption of debt	$ 16,000,000
Gain (loss) on early extinguishment of debt			$ (21,000,000)
6.3% senior notes due 2016
Debt Instrument [Line Items]
Stated interest rate		6.30%		6.30%
Repurchase amount		$ 500,000,000		$ 500,000,000
7.875% senior notes due 2020
Debt Instrument [Line Items]
Stated interest rate		7.875%		7.875%
Repurchase amount		$ 250,000,000		$ 250,000,000
5.875% senior notes due 2021
Debt Instrument [Line Items]
Stated interest rate		5.875%		5.875%
Repurchase amount		$ 350,000,000		$ 350,000,000
8.375% senior notes due 2018
Debt Instrument [Line Items]
Stated interest rate					8.375%
Repurchase amount					$ 350,000,000
Aggregate consideration					$ 377,000,000