Debt and Credit Facilities - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Sep. 30, 2020	Mar. 31, 2020
Debt Instrument [Line Items]
Debt, Face value	$ 106,209	$ 98,074
Debt, Carrying value	$ 105,902	$ 97,740
Weighted average contractual interest rates	1.56%	2.08%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face value	$ 85,685	$ 83,477
Debt, Carrying value	$ 85,422	$ 83,172
Weighted average contractual interest rates	1.54%	2.07%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face value	$ 20,524	$ 14,597
Debt, Carrying value	$ 20,480	$ 14,568
Weighted average contractual interest rates	1.66%	2.13%