Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at the federal statutory rate			$ (93)	$ (192)
State income taxes, net of federal income tax effect			(34)	(38)
Cumulative effect of changes in corporate state income tax laws that were recognized within State income taxes, net of federal income tax effect			59	(27)
Tax benefit from organizational restructuring			13	0
Increase deferred tax liability for business activity changes			0	(65)
Credit for increasing research activities			11	8
Change in federal and state valuation allowance			11	(60)
Other, net			(5)	(16)
Income tax expense	$ (17)	$ (98)	$ (64)	$ (390)
Effective income tax rate			(14.40%)	(71.20%)