Debt - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Debt Instrument [Line Items]
Debt	$ 98,233	$ 93,594
Weighted average contractual interest rates	1.60%	1.30%
Commercial paper [Member]
Debt Instrument [Line Items]
Debt	$ 26,632	$ 26,608
Weighted average contractual interest rates	1.11%	0.60%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 57,282	$ 52,863
Weighted average contractual interest rates	1.91%	1.76%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 14,319	$ 14,123
Weighted average contractual interest rates	1.32%	0.91%