Leases - Supplemental Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2019	Dec. 31, 2019
Leases [Abstract]
Operating cash flows from finance leases	$ 2	$ 5
Operating cash flows from operating leases	579	1,756
Financing cash flows from finance leases	1	2
Total cash paid for amounts included in the measurement of lease liabilities	582	1,763
Operating lease right-of-use assets obtained in exchange for lease obligations:	$ 262	$ 643