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Debt (Components of Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Apr. 07, 2020	Dec. 31, 2019	Jul. 31, 2018	Jan. 31, 2015
Debt Instrument [Line Items]
Debt Instrument	$ 3,171		$ 2,262
Unamortized Issuance Expense	(20)		(19)
Unamortized Debt Discount	(1)		(1)
Total	3,150		2,242
Debt issuance costs, line of credit	$ 12		11
Line of Credit \| 2018 Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, variable interest rate	2.11%
Line of Credit \| 2018 Revolving Credit Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt Instrument	$ 700
Unamortized Issuance Expense	0
Unamortized Debt Discount	0
Total	700
Senior Notes \| 4.10% Senior Notes Due January March 2022
Debt Instrument [Line Items]
Debt Instrument	207		213
Unamortized Issuance Expense	0		(1)
Unamortized Debt Discount	0		0
Total	$ 207		$ 212
Stated interest rate	4.10%		4.10%
Senior Notes \| 4.95% Senior Notes Due January 2025
Debt Instrument [Line Items]
Debt Instrument	$ 856		$ 892
Unamortized Issuance Expense	(4)		(5)
Unamortized Debt Discount	(1)		(1)
Total	$ 851		$ 886
Stated interest rate	4.95%	6.45%	4.95%	6.20%	4.95%
Senior Notes \| 7.50% Senior Notes Due April 2026
Debt Instrument [Line Items]
Debt Instrument	$ 618		$ 639
Unamortized Issuance Expense	(6)		(7)
Unamortized Debt Discount	0		0
Total	$ 612		$ 632
Stated interest rate	7.50%		7.50%
Senior Notes \| 7.75% Senior Notes Due October 2027
Debt Instrument [Line Items]
Debt Instrument	$ 440		$ 484
Unamortized Issuance Expense	(5)		(6)
Unamortized Debt Discount	0		0
Total	$ 435		$ 478
Stated interest rate	7.75%		7.75%
Senior Notes \| Eight Point Three Seven Five Percent Senior Notes Due September 2028
Debt Instrument [Line Items]
Debt Instrument	$ 350
Unamortized Issuance Expense	(5)
Unamortized Debt Discount	0
Total	$ 345
Stated interest rate	8.375%
Term Loan \| 2018 Term Loan Facility Due April 2023
Debt Instrument [Line Items]
Debt Instrument			$ 34
Unamortized Issuance Expense			0
Unamortized Debt Discount			0
Total			$ 34
Debt variable rate			4.31%