Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	26.90%	9.40%	19.60%	20.70%
United States statutory tax rate	35.00%	35.00%	35.00%	35.00%
Excess Tax Benefit from Share-based Compensation, Financing Activities	$ 4	$ 25	$ 37	$ 65
Unrecognized tax benefits, period increase (decrease)	10		18
Gross unrecognized tax benefits	349		349
Amount of unrecognized tax benefits that would affect the effective tax rate	322		322
The total amount of unrecognized tax benefits that, if recognized, would result in adjustments to deferred tax assets with corresponding adjustments to the valuation allowance			28
Unrecognized tax benefits income tax penalties and interest accrued increase (decrease)	1		2
Combined amount of accrued interest and penalties related to uncertain tax positions	16		16
Maximum
Income Tax Footnote Disclosure [Line Items]
Amount of unrecognized tax benefits for which it is reasonably possible that there will be a reduction within the next 12 months	$ 67		$ 67