Debt and Credit Facilities - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Dec. 31, 2019	Mar. 31, 2019
Debt Instrument [Line Items]
Debt, Face Value	$ 97,899	$ 93,296
Debt, Carrying Value	$ 97,534	$ 92,922
Weighted average contractual interest rates	2.30%	2.60%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face Value	$ 84,528	$ 80,875
Debt, Carrying Value	$ 84,190	$ 80,521
Weighted average contractual interest rates	2.28%	2.60%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face Value	$ 13,371	$ 12,421
Debt, Carrying Value	$ 13,344	$ 12,401
Weighted average contractual interest rates	2.40%	2.62%