Debt Obligations (Changes in Debt Obligations) (Details) - USD ($)	1 Months Ended		5 Months Ended	9 Months Ended
	Jun. 30, 2017	May 31, 2017	Sep. 30, 2017	Sep. 30, 2017	Apr. 30, 2017	Mar. 31, 2016
Changes In Short-Term Debt Obligations [Roll Forward]
Short-term debt - commercial paper beginning balance				$ 0
Short-term debt - commercial paper ending balance			$ 154,000,000	154,000,000
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				3,603,000,000
Additions				513,000,000
Payments, Accretion and Other				(527,000,000)
Long-term debt obligations at end of period			3,589,000,000	3,589,000,000
Changes In Debt Obligations [Roll Forward]
Total debt obligations at beginning of period				3,603,000,000
Additions				1,915,000,000
Payments, Accretion and Other				(1,775,000,000)
Total debt obligations at end of period			3,743,000,000	3,743,000,000
$400 million senior unsecured term loan facility due November 25, 2019 (average interest rate of 2.43% for the period January 1, 2017 through September 30, 2017)
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				399,000,000
Additions				0
Payments, Accretion and Other	$ (300,000,000)			(299,000,000)
Long-term debt obligations at end of period			100,000,000	100,000,000
Credit facility, borrowing capacity			400,000,000	$ 400,000,000		$ 400,000,000
Interest rate during period				2.43%
Short-term debt - commercial paper
Changes In Short-Term Debt Obligations [Roll Forward]
Short-term debt - commercial paper beginning balance				$ 0
Additions				1,402,000,000
Payments, Accretion and Other				(1,248,000,000)
Short-term debt - commercial paper ending balance			154,000,000	154,000,000
Senior Notes | 5.55% senior unsecured notes due January 15, 2020
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				598,000,000
Additions				0
Payments, Accretion and Other				0
Long-term debt obligations at end of period			$ 598,000,000	$ 598,000,000
Stated rate			5.55%	5.55%
Senior Notes | 5.25% senior unsecured notes repaid on May 26, 2017
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 369,000,000
Additions				0
Payments, Accretion and Other		$ (370,000,000)		(369,000,000)
Long-term debt obligations at end of period			$ 0	$ 0
Stated rate		5.25%	5.25%	5.25%
Senior Notes | 3.875% senior unsecured notes due June 7, 2021
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 625,000,000
Additions				0
Payments, Accretion and Other				80,000,000
Long-term debt obligations at end of period			$ 705,000,000	$ 705,000,000
Stated rate			3.875%	3.875%
Senior Notes | 4.25% senior unsecured notes due June 1, 2024
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 495,000,000
Additions				0
Payments, Accretion and Other				1,000,000
Long-term debt obligations at end of period			$ 496,000,000	$ 496,000,000
Stated rate			4.25%	4.25%
Senior Notes | 1.75% senior unsecured notes due May 19, 2023
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 622,000,000
Additions				0
Payments, Accretion and Other				79,000,000
Long-term debt obligations at end of period			$ 701,000,000	$ 701,000,000
Stated rate			1.75%	1.75%
Senior Notes | 3.85% senior unsecured notes due June 30, 2026
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 495,000,000
Additions				0
Payments, Accretion and Other				1,000,000
Long-term debt obligations at end of period			$ 496,000,000	$ 496,000,000
Stated rate			3.85%	3.85%
Senior Notes | Senior unsecured floating rate notes due March 22, 2019
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 0
Additions				498,000,000
Payments, Accretion and Other				0
Long-term debt obligations at end of period			$ 498,000,000	$ 498,000,000
Senior Notes | Senior unsecured floating rate notes due March 22, 2019 | LIBOR
Changes in Long-Term Debt Obligations [Roll Forward]
Spread on variable rate				0.39%
Revolving Credit Facility | $1 billion revolving credit commitment due April 25, 2022 (average interest rate of 2.40% for the period April 25, 2017 through September 30, 2017)
Changes in Long-Term Debt Obligations [Roll Forward]
Long-term debt obligations at beginning of period				$ 0
Additions				15,000,000
Payments, Accretion and Other				(20,000,000)
Long-term debt obligations at end of period			(5,000,000)	(5,000,000)
Credit facility, borrowing capacity			$ 1,000,000,000	$ 1,000,000,000	$ 1,000,000,000
Interest rate during period			2.40%