Consolidating Financial Information - Narrative (Details) - Senior Notes - USD ($)	Jun. 30, 2018	Dec. 31, 2017
8.75% due 2020
Debt Instrument [Line Items]
Debt instrument, face amount	$ 282,000,000
Interest rate, stated percentage	8.75%	8.75%
5.125% due 2023
Debt Instrument [Line Items]
Debt instrument, face amount	$ 1,000,000,000
Interest rate, stated percentage	5.125%	5.125%
5% due 2026
Debt Instrument [Line Items]
Debt instrument, face amount	$ 900,000,000
Interest rate, stated percentage	5.00%	5.00%
4.875% due 2027
Debt Instrument [Line Items]
Debt instrument, face amount	$ 700,000,000
Interest rate, stated percentage	4.875%	4.875%