Leases - Components of Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 156	$ 166	$ 463	$ 506
Short-term lease cost	14	17	40	41
Variable lease cost	17	9	40	35
Sublease income	(10)	(9)	(32)	(28)
Total operating costs	177	183	511	554
Finance lease cost:
Amortization of right-of-use assets	107	90	322	339
Interest on lease liabilities	10	16	35	50
Total finance lease cost	$ 117	$ 106	$ 357	$ 389