Leases - Components of Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating lease expense	$ 547	$ 1,077
Amortization of right-to-use assets	19	37
Interest on lease liabilities	0	1
Total finance lease expense	19	38
Variable lease expense	27	42
Total lease expense	$ 593	$ 1,157