Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 4,851	$ 5,013	$ 9,741	$ 10,295
United States
Disaggregation of Revenue [Line Items]
Revenues	1,806	1,863	3,657	3,750
United Kingdom
Disaggregation of Revenue [Line Items]
Revenues	678	760	1,393	1,560
Australia
Disaggregation of Revenue [Line Items]
Revenues	366	391	739	845
Other Europe
Disaggregation of Revenue [Line Items]
Revenues	1,260	1,263	2,490	2,610
Other International
Disaggregation of Revenue [Line Items]
Revenues	$ 741	$ 736	$ 1,462	$ 1,530