Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Accounting Policies [Abstract]
Amortization period for capitalized contract costs	5 years		5 years
Capitalized contract costs within other current assets	$ 2		$ 2
Capitalized contract costs within other assets	7		7
Amortization of capitalized contract cost	1		1
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net of accumulated amortization	1,441		1,441		$ 897
Amortization expense	44	$ 16	83	$ 36
Outsourcing contract costs
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net of accumulated amortization	13		13		$ 19
Amortization expense	$ 1		$ 6