Debt - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Dec. 31, 2016	Mar. 31, 2016
Debt Instrument [Line Items]
Debt	$ 96,198	$ 93,594
Weighted average contractual interest rates	1.52%	1.30%
Commercial paper [Member]
Debt Instrument [Line Items]
Debt	$ 29,023	$ 26,608
Weighted average contractual interest rates	0.99%	0.60%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 54,101	$ 52,741
Weighted average contractual interest rates	1.88%	1.76%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 12,990	$ 14,123
Weighted average contractual interest rates	1.21%	0.91%
Carrying value adjustment [Member]
Debt Instrument [Line Items]
Debt	$ 84	$ 122