Debt - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Debt Instrument [Line Items]
Commercial paper	$ 27,313	$ 26,632
Debt	$ 98,353	$ 98,233
Weighted average contractual interest rates	2.04%	1.60%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 57,402	$ 57,282
Weighted average contractual interest rates	2.18%	1.91%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt	$ 13,638	$ 14,319
Weighted average contractual interest rates	1.95%	1.32%
Commercial paper [Member]
Debt Instrument [Line Items]
Weighted average contractual interest rates	1.80%	1.11%