Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 19,577	$ 20,753	$ 21,733
United States
Disaggregation of Revenue [Line Items]
Revenues	7,225	7,677	8,015
United Kingdom
Disaggregation of Revenue [Line Items]
Revenues	2,776	3,175	3,392
Australia
Disaggregation of Revenue [Line Items]
Revenues	1,487	1,582	1,694
Other Europe
Disaggregation of Revenue [Line Items]
Revenues	5,121	5,294	5,409
Other International
Disaggregation of Revenue [Line Items]
Revenues	$ 2,968	$ 3,025	$ 3,223