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Postretirement Benefits - Schedule Of Net Periodic Expense (Income) For Postretirement Welfare Benefits (Details) - Other Postretirement Benefits Plan - USD ($)
$ in Millions
 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 1 $ 1 $ 1
Interest cost 2 2 2
Amortization of unrecognized actuarial gain and prior service credit (5) (6) (1)
Net periodic pension expense $ (2) $ (3) $ 2