Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 4,554	$ 4,851	$ 9,056	$ 9,741
United States
Disaggregation of Revenue [Line Items]
Revenues	1,666	1,806	3,375	3,657
United Kingdom
Disaggregation of Revenue [Line Items]
Revenues	579	678	1,152	1,393
Other Europe
Disaggregation of Revenue [Line Items]
Revenues	1,244	1,260	2,449	2,490
Australia
Disaggregation of Revenue [Line Items]
Revenues	390	366	751	739
Other International
Disaggregation of Revenue [Line Items]
Revenues	$ 675	$ 741	$ 1,329	$ 1,462