Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Amortization period for capitalized contract costs	5 years		5 years
Capitalized contract costs within other current assets	$ 3		$ 3
Capitalized contract costs within other assets	6		6
Amortization of capitalized contract cost	1		2
Intangible assets, net of accumulated amortization	1,406		1,406		$ 897
Amortization expense	43	$ 18	126	$ 54
Outsourcing contract costs
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net of accumulated amortization	15		15		$ 19
Amortization expense	$ (1)		$ 5