Postretirement Benefits - Schedule Of Changes In Benefit Obligations For Postretirement Welfare Plans (Details) - Other Postretirement Benefits Plan - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Change in benefit obligations
Benefit obligation at beginning of period	$ 78	$ 82
Service cost	1	1	$ 1
Interest cost	2	2	2
Actuarial gain	(3)	(1)
Benefits paid	(5)	(6)
Benefit obligation at end of period	$ 73	$ 78	$ 82