Income Taxes (Unrecognized Tax Benefit Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	$ 239	$ 190
Additions based on current year tax positions	17	21
Additions based on prior year tax positions	12	53
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	23	24
Reductions for lapse of statute of limitations	(3)	(1)
Balance at end of period	$ 242	$ 239