Income Taxes (Income Tax (Expense) Benefit Allocated to Other Items) (Details) - USD ($) $ in Millions		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Income Tax Contingency [Line Items]
Unrecognized net periodic pension and other postretirement benefit cost	[1]	$ (24)	$ 0	$ 0

[1] (1)These amounts have been recognized in accumulated other comprehensive loss.