Income Taxes (Income Tax (Expense) Benefit Allocated to Other Items) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2013
Income Tax Contingency [Line Items]
Unrecognized net periodic pension and other postretirement benefit cost	$ 9	$ (24)	$ 0
Successor [Member]
Income Tax Contingency [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	$ (6)		$ 0	$ 0