Debt - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Debt	$ 98,776	$ 98,233
Weighted average contractual interest rates	1.67%	1.60%
Commercial paper [Member]
Debt Instrument [Line Items]
Debt	$ 26,498	$ 26,632
Weighted average contractual interest rates	1.18%	1.11%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 58,465	$ 57,282
Weighted average contractual interest rates	1.94%	1.91%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 13,813	$ 14,319
Weighted average contractual interest rates	1.47%	1.32%