Debt Senior Unsecured Notes (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2015	Sep. 30, 2014
Debt Instrument [Line Items]
Long-term Debt		$ 11,857	$ 11,857	$ 10,156
Debt Instrument, Interest Rate, Effective Percentage		5.80%	5.80%	5.70%
Debt Issuance Costs, Net		$ 114	$ 114
Gains (Losses) on Extinguishment of Debt			(58)	$ 28	$ (38)
Senior Unsecured Notes 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 623	$ 623	$ 1,227
Debt Instrument, Interest Rate, Effective Percentage	[2]	5.70%	5.70%	5.60%
Debt Instrument, Face Amount							$ 1,250
Debt Instrument, Interest Rate, Stated Percentage							5.375%
Gains (Losses) on Extinguishment of Debt		$ 24	$ 0	$ 1
Senior Unsecured Notes 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 479	$ 479	$ 599
Debt Instrument, Interest Rate, Effective Percentage	[2]	5.70%	5.70%	5.70%
Debt Instrument, Face Amount						$ 650
Debt Instrument, Interest Rate, Stated Percentage						5.50%
Gains (Losses) on Extinguishment of Debt			$ (1)	$ 4
Senior Unsecured Notes 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 1,174	$ 1,174	$ 1,210
Debt Instrument, Interest Rate, Effective Percentage	[2]	5.80%	5.80%	6.00%
Debt Instrument, Face Amount							$ 1,550
Debt Instrument, Interest Rate, Stated Percentage							5.75%
Gains (Losses) on Extinguishment of Debt			$ 3	$ 30
Senior Unsecured Notes 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 1,387	$ 1,387	$ 0
Debt Instrument, Interest Rate, Effective Percentage	[2]	5.30%	5.30%	0.00%
Debt Instrument, Face Amount		$ 1,400	$ 1,400
Debt Instrument, Interest Rate, Stated Percentage		5.125%	5.125%
Debt Issuance Costs, Net		$ 13	$ 13
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 3,663	3,663	$ 3,036
Gains (Losses) on Extinguishment of Debt			$ 2	$ 35

[1] On December 27, 2019, we used the proceeds from the issuance of our 2028 Senior Unsecured Notes (discussed below) to redeem approximately $613 million in aggregate principal amount of our 2023 Senior Unsecured Notes, plus accrued and unpaid interest. On January 21, 2020, we redeemed the remaining $623 million in aggregate principal amount of our 2023 Senior Unsecured Notes, which was included in debt, current portion on our Consolidated Balance Sheet at December 31, 2019, with the proceeds from the 2028 Senior Unsecured Notes, which was included in cash and cash equivalents on our Consolidated Balance Sheet at December 31, 2019. We recorded approximately $24 million in loss on extinguishment of debt which is comprised of approximately $18 million of prepayment premiums and approximately $6 million associated with the write-off of unamortized debt issuance costs during the fourth quarter of 2019 associated with the redemption.

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