Debt Debt (First Lien Notes) (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Debt Instrument [Line Items]
Gains (Losses) on Extinguishment of Debt			$ 28	$ (38)	$ (25)
Long-term Debt			10,156	11,213
Debt Issuance Costs, Net			112
Long-term Debt, Gross			10,918
2022 First Lien Notes [Member]
Debt Instrument [Line Items]
Long-term Debt			$ 743	$ 741
Debt Instrument, Interest Rate, Effective Percentage	[1]		6.40%	6.40%
New 2019 First Lien Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 400
Debt Issuance Costs, Net		$ 8
2024 First Lien Notes [Member]
Debt Instrument [Line Items]
Long-term Debt			$ 486	$ 485
Debt Instrument, Interest Rate, Effective Percentage	[1]		6.10%	6.10%
2026 First Lien Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 560		$ 625
Long-term Debt			$ 1,171	$ 1,170
Debt Instrument, Interest Rate, Effective Percentage	[1]		5.50%	5.50%
Debt Instrument, Interest Rate, Stated Percentage				5.25%		5.25%
Debt Issuance Costs, Net				$ 8		$ 9
Corporate Debt Securities [Member]
Debt Instrument [Line Items]
Long-term Debt			$ 2,400	$ 2,396
London Interbank Offered Rate (LIBOR) [Member] | New 2019 First Lien Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.75%

[1] Our weighted average interest rate calculation includes the amortization of debt issuance costs and debt discount.