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The average effective interest rate during the reporting period.

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Debt (Components of Debt) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Debt (Components of Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Jun. 30, 2019	Apr. 07, 2020	Dec. 31, 2019	Jul. 31, 2018
Debt Instrument [Line Items]
Debt Instrument	$ 2,297			$ 2,262
Unamortized Issuance Expense	(17)			(19)
Unamortized Debt Discount	(1)			(1)
Total debt	2,279			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	149
Unamortized Issuance Expense	0
Unamortized Debt Discount	0
Total debt	$ 149
Variable interest rate	2.12%
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	$ 210			213
Unamortized Issuance Expense	(1)			(1)
Unamortized Debt Discount	0			0
Total debt	$ 209			$ 212
Stated interest rate	4.10%			4.10%
Senior Notes \| 4.95% Senior Notes due January 2025
Debt Instrument [Line Items]
Debt Instrument	$ 864			$ 892
Unamortized Issuance Expense	(5)			(5)
Unamortized Debt Discount	(1)			(1)
Total debt	$ 858			$ 886
Stated interest rate	4.95%			4.95%	6.20%
Senior Notes \| 4.95% Senior Notes due January 2025 \| Subsequent event
Debt Instrument [Line Items]
Stated interest rate			6.45%
Senior Notes \| 7.50% Senior Notes due April 2026
Debt Instrument [Line Items]
Debt Instrument	$ 621			$ 639
Unamortized Issuance Expense	(6)			(7)
Unamortized Debt Discount	0			0
Total debt	$ 615			$ 632
Stated interest rate	7.50%			7.50%
Senior Notes \| 7.75% Senior Notes due October 2027
Debt Instrument [Line Items]
Debt Instrument	$ 453			$ 484
Unamortized Issuance Expense	(5)			(6)
Unamortized Debt Discount	0			0
Total debt	$ 448			$ 478
Stated interest rate	7.75%			7.75%
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)