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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	6 Months Ended	9 Months Ended
Debt (Components of Debt) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019	Apr. 07, 2020	Dec. 31, 2019	Jan. 31, 2015
Debt Instrument [Line Items]
Debt Instrument	$ 2,457			$ 2,262
Unamortized Issuance Expense	(16)			(19)
Unamortized Debt Discount	(1)			(1)
Total debt	2,440			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	336
Unamortized Issuance Expense	0
Unamortized Debt Discount	0
Total debt	$ 336
Variable interest rate	1.86%
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	(1)			(1)
Unamortized Debt Discount	0			0
Total debt	$ 206			$ 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	6.20%		6.45%	4.95%	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%
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)