Postretirement Benefits - Schedule Of Changes In Benefit Obligations For Postretirement Welfare Plans (Details) - Other Postretirement Benefit Plan, Defined Benefit - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Change in benefit obligations
Benefit obligation at beginning of period	$ 118	$ 119
Service cost	1	1	$ 2
Interest cost	4	5	5
Plan amendments	(16)	0
Actuarial loss	3	5
Benefit payments	(11)	(12)
Curtailment gain	(1)	0
Benefit obligation at end of period	$ 98	$ 118	$ 119