Postretirement Benefits - Schedule Of Changes In Benefit Obligations For Postretirement Welfare Plans (Details) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Change in benefit obligations
Benefit obligation at beginning of period	$ 98	$ 118
Service cost	1	1	$ 1
Interest cost	2	4	5
Plan amendments	0	(16)
Actuarial (gain) / loss	(13)	3
Benefits paid	(6)	(11)
Curtailment gain	0	(1)
Benefit obligation at end of period	$ 82	$ 98	$ 118