Debt and Financing Activities - Long-Term Debt (Details) - USD ($)		12 Months Ended
	Mar. 05, 2014	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Debt Instrument [Line Items]
Total debt outstanding		$ 8,147,000,000	$ 9,709,000,000
Net proceeds from issuance of notes	$ 4,068,000,000	$ 0	3,000,000	$ 4,124,000,000
Days notice for redemption prior to maturity		30 days
Redemption purchase offer		101.00%
Floating Rate Notes
Debt Instrument [Line Items]
Total debt outstanding		$ 0	400,000,000
Aggregate principal amount	400,000,000
2017 Notes
Debt Instrument [Line Items]
Total debt outstanding		$ 700,000,000	700,000,000
Aggregate principal amount	$ 700,000,000
Debt interest rate	1.29%	1.29%
2019 Notes
Debt Instrument [Line Items]
Total debt outstanding		$ 1,100,000,000	1,100,000,000
Aggregate principal amount	$ 1,100,000,000
Debt interest rate	2.28%	2.28%
2024 Notes
Debt Instrument [Line Items]
Total debt outstanding		$ 1,100,000,000	1,100,000,000
Aggregate principal amount	$ 1,100,000,000
Debt interest rate	3.80%	3.80%
2044 Notes
Debt Instrument [Line Items]
Total debt outstanding		$ 800,000,000	800,000,000
Aggregate principal amount	$ 800,000,000
Debt interest rate	4.88%	4.88%
Long-term Debt, Current Maturities
Debt Instrument [Line Items]
Total debt outstanding		$ 1,612,000,000	$ 1,529,000,000