Leases - Supplemental Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating cash flows from finance leases	$ 1	$ 3
Operating cash flows from operating leases	601	1,177
Financing cash flows from finance leases	0	1
Total cash paid for amounts included in the measurement of lease liabilities	602	1,181
Operating lease right-of-use assets obtained in exchange for lease obligations:	$ 268	$ 381