Leases - Components of lease costs (Details) $ in Millions	12 Months Ended
	Mar. 31, 2020 USD ($)
Leases [Abstract]
Short-term lease cost	$ 29
Operating lease cost	459
Amortization of right-of-use assets	14
Interest on lease liabilities	5
Total finance lease cost	19
Variable lease cost	125
Sublease income	(33)
Total lease cost	$ 599