Postretirement Benefits (Schedule Of Net Periodic Expense (Income) For Postretirement Welfare Benefits) (Details) (Other Postretirement Benefit Plan, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 2	$ 2
Interest cost	5	5	6
Amortization of unrecognized actuarial gain and prior service costs	(4)	(1)	(2)
Curtailment gain	0	(2)	0
Net periodic expense (income)	$ 2	$ 4	$ 6