Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at the federal statutory rate			$ (192)	$ 94
State income taxes, net of federal income tax effect			(38)	5
Cumulative effect of changes in corporate state income tax laws that were recognized within State income taxes, net of federal income tax effect			(27)	3
Increase deferred tax liability for business activity changes			(65)	0
Credit for increasing research activities			8	0
Tax benefit from organizational restructuring			0	(42)
Change in federal and state valuation allowance			(60)	(314)
Other, net			(16)	(5)
Income tax expense	$ (98)	$ (119)	$ (390)	$ (175)
Effective income tax rate			(71.20%)	65.10%