Income Taxes (Income Tax (Expense) Benefit Allocated to Other Items) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Income Tax Contingency [Line Items]
Unrecognized net periodic pension and other postretirement benefit cost	$ 12	$ 9	$ (24)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	6	$ (6)	$ 0
Unrealized Gain (Loss) on Securities	$ 7