Postretirement Benefits - Schedule Of Net Periodic Expense (Income) For Postretirement Welfare Benefits (Details) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1	$ 1
Interest cost	2	4	5
Amortization of unrecognized actuarial gain and prior service credit	(1)	0	(4)
Net periodic expense	$ 2	$ 5	$ 2