Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	7.60%	42.10%	7.90%	12.50%
Provision for income taxes	$ 21	$ (16)	$ 47	$ 89
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	$ 16		$ 16
Excluding excess tax benefit [Member]
Income Tax Footnote Disclosure [Line Items]
Effective tax rates	9.80%		10.60%
Including U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Footnote Disclosure [Line Items]
United States statutory tax rate	21.00%		21.00%
Total U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Footnote Disclosure [Line Items]
Provision for income taxes					$ 235
Transition tax [Member]
Income Tax Footnote Disclosure [Line Items]
Provision for income taxes					$ 192