Debt (Components of Debt) (Details) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2016	Feb. 29, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jul. 31, 2018	Jul. 31, 2016	Jan. 31, 2015
Debt Instrument [Line Items]
Debt Instrument, long-term debt	$ 2,240,000,000				$ 2,240,000,000		$ 2,240,000,000
Debt Instrument	2,292,000,000	$ 2,342,000,000			2,292,000,000		2,292,000,000		$ 2,342,000,000
Unamortized Issuance Expense	(20,000,000)	(23,000,000)			(20,000,000)		(20,000,000)		(23,000,000)
Unamortized Debt Discount	(1,000,000)	(1,000,000)			(1,000,000)		(1,000,000)		(1,000,000)
Total, current portion of long-term debt	52,000,000	0			52,000,000		52,000,000		0
Total, long-term debt	2,219,000,000	2,318,000,000			2,219,000,000		2,219,000,000		2,318,000,000
Total debt	2,271,000,000	2,318,000,000			2,271,000,000		2,271,000,000		2,318,000,000
Unamortized issuance expense	20,000,000	23,000,000			20,000,000		20,000,000		23,000,000
Repayments of long-term debt							43,000,000	$ 1,191,000,000
Decrease in outstanding debt					50,000,000
Gain on extinguishment of debt					7,000,000	$ 0	7,000,000	$ (8,000,000)
Line of Credit | 2018 revolving credit facility, due April 2023
Debt Instrument [Line Items]
Debt Instrument, long-term debt	0				0		0
Unamortized Issuance Expense	0				0		0
Unamortized Debt Discount	0				0		0
Total, long-term debt	0				0		$ 0
Variable interest rate							3.42%
Unamortized issuance expense	0				0		$ 0
Line of Credit | 2018 term loan facility, due April 2023
Debt Instrument [Line Items]
Debt Instrument		0							0
Unamortized Issuance Expense		0							0
Unamortized Debt Discount		0							0
Total debt		0							$ 0
Variable interest rate									3.92%
Unamortized issuance expense		0							$ 0
Senior Notes
Debt Instrument [Line Items]
Debt Instrument, current portion of long-term debt	52,000,000				52,000,000		52,000,000
Unamortized Issuance Expense	0				0		0
Unamortized Debt Discount	0				0		0
Total, current portion of long-term debt	52,000,000				52,000,000		52,000,000
Unamortized issuance expense	0				0		0
Repayments of long-term debt		900,000,000			43,000,000
Gain on extinguishment of debt	7,000,000
Senior Notes | 4.05% Senior Notes due January 2020
Debt Instrument [Line Items]
Debt Instrument, current portion of long-term debt	52,000,000				52,000,000		52,000,000
Debt Instrument		52,000,000							52,000,000
Unamortized Issuance Expense	0	0			0		0		0
Unamortized Debt Discount	0	0			0		0		0
Total, current portion of long-term debt	$ 52,000,000				$ 52,000,000		$ 52,000,000
Total debt		$ 52,000,000							$ 52,000,000
Stated interest rate	4.05%	4.05%			4.05%		4.05%		4.05%
Unamortized issuance expense	$ 0	$ 0			$ 0		$ 0		$ 0
Senior Notes | 4.10% Senior Notes due March 2022
Debt Instrument [Line Items]
Debt Instrument, long-term debt	213,000,000				213,000,000		213,000,000
Debt Instrument		213,000,000							213,000,000
Unamortized Issuance Expense	(1,000,000)	(1,000,000)			(1,000,000)		(1,000,000)		(1,000,000)
Unamortized Debt Discount	0	0			0		0		0
Total, long-term debt	$ 212,000,000				$ 212,000,000		$ 212,000,000
Total debt		$ 212,000,000							$ 212,000,000
Stated interest rate	4.10%	4.10%			4.10%		4.10%		4.10%
Unamortized issuance expense	$ 1,000,000	$ 1,000,000			$ 1,000,000		$ 1,000,000		$ 1,000,000
Senior Notes | 4.95% Senior Notes due January 2025
Debt Instrument [Line Items]
Debt Instrument, long-term debt	896,000,000				896,000,000		896,000,000
Debt Instrument		927,000,000							927,000,000
Unamortized Issuance Expense	(7,000,000)	(7,000,000)			(7,000,000)		(7,000,000)		(7,000,000)
Unamortized Debt Discount	(1,000,000)	(1,000,000)			(1,000,000)		(1,000,000)		(1,000,000)
Total, long-term debt	$ 888,000,000				$ 888,000,000		$ 888,000,000
Total debt		$ 919,000,000							$ 919,000,000
Stated interest rate	4.95%	4.95%			4.95%		4.95%		4.95%
Unamortized issuance expense	$ 7,000,000	$ 7,000,000			$ 7,000,000		$ 7,000,000		$ 7,000,000
Repurchased amount	31,000,000				31,000,000		31,000,000
Senior Notes | 7.50% Senior Notes due April 2026
Debt Instrument [Line Items]
Debt Instrument, long-term debt	639,000,000				639,000,000		639,000,000
Debt Instrument		650,000,000							650,000,000
Unamortized Issuance Expense	(7,000,000)	(8,000,000)			(7,000,000)		(7,000,000)		(8,000,000)
Unamortized Debt Discount	0	0			0		0		0
Total, long-term debt	$ 632,000,000				$ 632,000,000		$ 632,000,000
Total debt		$ 642,000,000							$ 642,000,000
Stated interest rate	7.50%	7.50%			7.50%		7.50%		7.50%
Unamortized issuance expense	$ 7,000,000	$ 8,000,000			$ 7,000,000		$ 7,000,000		$ 8,000,000
Repurchased amount	11,000,000				11,000,000		11,000,000
Senior Notes | 7.75% Senior Notes due October 2027
Debt Instrument [Line Items]
Debt Instrument, long-term debt	492,000,000				492,000,000		492,000,000
Debt Instrument		500,000,000							500,000,000
Unamortized Issuance Expense	(5,000,000)	(7,000,000)			(5,000,000)		(5,000,000)		(7,000,000)
Unamortized Debt Discount	0	0			0		0		0
Total, long-term debt	$ 487,000,000				$ 487,000,000		$ 487,000,000
Total debt		$ 493,000,000							$ 493,000,000
Stated interest rate	7.75%	7.75%			7.75%		7.75%		7.75%
Unamortized issuance expense	$ 5,000,000	$ 7,000,000			$ 5,000,000		$ 5,000,000		$ 7,000,000
Repurchased amount	$ 8,000,000				$ 8,000,000		$ 8,000,000
Long-term debt | 4.05% Senior Notes due January 2020
Debt Instrument [Line Items]
Stated interest rate	6.05%				6.05%		6.05%			5.30%	5.80%	4.05%
Long-term debt | 4.95% Senior Notes due January 2025
Debt Instrument [Line Items]
Stated interest rate	6.95%				6.95%		6.95%			6.20%	6.70%	4.95%
LIBOR | Senior Notes
Debt Instrument [Line Items]
Increase in basis spread			1.75%	1.75%
Other long-term assets | Line of Credit | 2018 revolving credit facility, due April 2023
Debt Instrument [Line Items]
Unamortized Issuance Expense	$ (9,000,000)	(12,000,000)			$ (9,000,000)		$ (9,000,000)		(12,000,000)
Unamortized issuance expense	$ 9,000,000	$ 12,000,000			$ 9,000,000		$ 9,000,000		$ 12,000,000