Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	42.10%	26.90%	12.50%	19.60%
United States statutory tax rate	35.00%		35.00%
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	$ 55		$ 55
Increase to net cash provided by Operating Activities [Member]
Income Tax Footnote Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities				$ 37
Excluding excess tax benefit [Member]
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	34.20%		18.40%
Provision for income taxes [Member] | Increase to net cash provided by Operating Activities [Member]
Income Tax Footnote Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities	$ 3		$ 42