Debt and Credit Facilities - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Sep. 30, 2021	Mar. 31, 2021
Debt Instrument [Line Items]
Debt, Face value	$ 112,000	$ 110,015
Debt, Carrying value	$ 111,709	$ 109,725
Weighted average contractual interest rates	1.11%	1.31%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face value	$ 84,934	$ 85,759
Debt, Carrying value	$ 84,689	$ 85,513
Weighted average contractual interest rates	1.15%	1.31%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face value	$ 27,066	$ 24,256
Debt, Carrying value	$ 27,020	$ 24,212
Weighted average contractual interest rates	1.01%	1.29%