Debt (Summary of Components of Debt) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Total principal amount	$ 6,300	$ 8,310
Less: unamortized discount and issuance costs	(98)	(124)
Total debt	6,202	8,186
Less: current portion	0	(1,310)	[1]
Total long-term portion	$ 6,202	6,876	[1]
Senior Notes | 2.75% Senior Notes due June 15, 2017
Debt Instrument [Line Items]
Stated interest rate (as a percent)	2.75%
Total principal amount	$ 0	600
Effective interest rate (as a percent)	2.79%
Senior Notes | 4.2% Senior Notes due September 15, 2020
Debt Instrument [Line Items]
Stated interest rate (as a percent)	4.20%
Total principal amount	$ 750	750
Effective interest rate (as a percent)	4.25%
Senior Notes | 3.95% Senior Notes due June 15, 2022
Debt Instrument [Line Items]
Stated interest rate (as a percent)	3.95%
Total principal amount	$ 400	400
Effective interest rate (as a percent)	4.05%
Senior Notes | 5.0% Senior Notes due April 15, 2025
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.00%
Total principal amount	$ 1,100	1,100
Effective interest rate (as a percent)	5.23%
Unsecured Debt | Senior Term Loan A-1 due May 10, 2019
Debt Instrument [Line Items]
Total principal amount	$ 800	1,000
Unsecured Debt | Senior Term Loan A-2 due August 1, 2019
Debt Instrument [Line Items]
Total principal amount	800	800
Unsecured Debt | Senior Term Loan A-3 due August 1, 2019
Debt Instrument [Line Items]
Total principal amount	200	200
Unsecured Debt | Senior Term Loan A-5 due August 1, 2021
Debt Instrument [Line Items]
Total principal amount	$ 500	1,710
Convertible Debt | 2.5% Convertible Senior Notes due April 1, 2021
Debt Instrument [Line Items]
Stated interest rate (as a percent)	2.50%
Total principal amount	$ 500	500
Effective interest rate (as a percent)	3.76%
Convertible Debt | 2.0% Convertible Senior Notes due August 15, 2021
Debt Instrument [Line Items]
Stated interest rate (as a percent)	2.00%
Total principal amount	$ 1,250	$ 1,250
Minimum | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Maximum | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%

[1] Derived from audited financial statements.