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Debt (Components of Debt) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Debt (Components of Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Apr. 07, 2020	Jul. 31, 2018	Jan. 31, 2015
Debt Instrument [Line Items]
Debt Instrument	$ 2,471	$ 2,262
Unamortized Issuance Expense	20	19
Unamortized Debt Discount	(1)	(1)
Total debt	2,450	2,242
Line of Credit \| Variable rate (2.120% at March 31, 2020) 2018 revolving credit facility, due April 2024
Debt Instrument [Line Items]
Debt Instrument	0
Unamortized Issuance Expense	0
Unamortized Debt Discount	0
Total debt	$ 0
Variable interest rate	1.60%
Line of Credit \| Variable rate (2.120% at March 31, 2020) 2018 revolving credit facility, due April 2024 \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt Instrument		34
Unamortized Issuance Expense		0
Unamortized Debt Discount		0
Total debt		$ 34
Line of Credit \| Variable rate (4.310% at December 31, 2019) 2018 term loan facility, due April 2024
Debt Instrument [Line Items]
Variable interest rate		4.31%
Senior Notes \| 4.10% Senior Notes due March 2022
Debt Instrument [Line Items]
Debt Instrument	$ 207	$ 213
Unamortized Issuance Expense	0	1
Unamortized Debt Discount	0	0
Total debt	$ 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 debt	851	$ 886
Stated interest rate		4.95%	6.45%	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 debt	$ 612	632
Stated interest rate	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 debt	$ 435	478
Stated interest rate	7.75%
Senior Notes \| 8.375% Senior Notes due September 2028 (3)
Debt Instrument [Line Items]
Debt Instrument	$ 350
Unamortized Issuance Expense	5
Unamortized Debt Discount	0
Total debt	$ 345
Stated interest rate	8.375%
Other long-term assets \| Line of Credit \| Variable rate (2.120% at March 31, 2020) 2018 revolving credit facility, due April 2024
Debt Instrument [Line Items]
Unamortized Issuance Expense	$ 10	$ 11