Debt Senior Unsecured Notes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2015	Sep. 30, 2014
Debt Instrument [Line Items]
Long-term Debt		$ 10,156	$ 11,213
Debt Issuance Costs, Net		112
Gains (Losses) on Extinguishment of Debt		28	(38)	$ (25)
Senior Unsecured Notes 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 1,227	$ 1,239
Debt Instrument, Interest Rate, Effective Percentage	[1]	5.60%	5.60%
Debt Instrument, Face Amount						$ 1,250
Debt Instrument, Interest Rate, Stated Percentage						5.375%
Gains (Losses) on Extinguishment of Debt		$ 1
Senior Unsecured Notes 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 599	$ 644
Debt Instrument, Interest Rate, Effective Percentage	[1]	5.70%	5.70%
Debt Instrument, Face Amount					$ 650
Debt Instrument, Interest Rate, Stated Percentage					5.50%
Gains (Losses) on Extinguishment of Debt		$ 4
Senior Unsecured Notes 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 1,210	$ 1,534
Debt Instrument, Interest Rate, Effective Percentage	[1]	6.00%	6.00%
Debt Instrument, Face Amount						$ 1,550
Debt Instrument, Interest Rate, Stated Percentage						5.75%
Gains (Losses) on Extinguishment of Debt		$ 30
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt		3,036	$ 3,417
Gains (Losses) on Extinguishment of Debt		$ 35

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