Debt and Credit Facilities - Debt and Related Weighted Average Contractual Interest Rates (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Debt Instrument [Line Items]
Debt, Face Value	$ 93,296	$ 98,699
Debt, Carrying Value	$ 92,922	$ 98,353
Weighted average contractual interest rates	2.60%	2.04%
Unsecured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face Value	$ 80,875	$ 85,042
Debt, Carrying Value	$ 80,521	$ 84,715
Weighted average contractual interest rates	2.60%	2.05%
Secured notes and loans payable [Member]
Debt Instrument [Line Items]
Debt, Face Value	$ 12,421	$ 13,657
Debt, Carrying Value	$ 12,401	$ 13,638
Weighted average contractual interest rates	2.62%	1.95%