Postretirement Benefits - Schedule Of Net Periodic Expense (Income) For Postretirement Welfare Benefits (Details) - Other Postretirement Benefit Plan, Defined Benefit - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1	$ 2
Interest cost	4	5	5
Amortization of unrecognized actuarial gain and prior service credit	0	(4)	(1)
Curtailment/settlement loss (gain)	0	0	(2)
Net periodic expense	$ 5	$ 2	$ 4