Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at the federal statutory rate			$ 101	$ (93)
State income taxes, net of federal income tax effect			18	(34)
Cumulative effect of changes in corporate state income tax laws that were recognized within State income taxes, net of federal income tax effect			(1)	59
Tax benefit from organizational restructuring			(4)	13
Credit for increasing research activities			6	11
Change in federal and state valuation allowance			(37)	11
Other, net			3	(5)
Income tax benefit (expense)	$ 64	$ (17)	$ 86	$ (64)
Effective income tax rate			(18.00%)	(14.40%)