Debt and Credit Facilities - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 31, 2020
Debt Instrument [Line Items]
Debt, Face value	$ 109,263	$ 98,074
Debt, Carrying value	$ 108,972	$ 97,740
Weighted average contractual interest rates	1.45%	2.08%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face value	$ 87,504	$ 83,477
Debt, Carrying value	$ 87,252	$ 83,172
Weighted average contractual interest rates	1.42%	2.07%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face value	$ 21,759	$ 14,597
Debt, Carrying value	$ 21,720	$ 14,568
Weighted average contractual interest rates	1.59%	2.13%