Income Taxes (Unrecognized Tax Benefit Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	$ 190	$ 166
Additions based on current year tax positions	21	15
Additions based on prior year tax positions	53	10
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	24		$ 0
Reductions for lapse of statute of limitations	(1)	(1)
Balance at end of period	$ 239	$ 190	$ 166