Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2019	Mar. 31, 2018
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	(1062.50%)	83.30%	37.30%	18.80%
Income Tax Expense (Benefit)	$ 170	$ (5)	$ 259	$ 93
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	$ 45		$ 45
Increase to net cash provided by Operating Activities [Member]
Income Tax Footnote Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities				$ 53
Excluding excess tax benefit [Member]
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	(1075.00%)		43.60%
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			$ 40
Excluding U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	37.50%		11.90%
Total U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Footnote Disclosure [Line Items]
Income Tax Expense (Benefit)	$ 176		$ 176
Transition tax [Member]
Income Tax Footnote Disclosure [Line Items]
Income Tax Expense (Benefit)	$ 151		$ 151
Subsequent Event [Member] | Including U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Footnote Disclosure [Line Items]
United States statutory tax rate					21.00%	31.55%