Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at the federal statutory rate	$ 29	$ (46)
State income taxes, net of federal income tax effect	5	(15)
Cumulative effect of changes in corporate state income tax laws that were recognized within State income taxes, net of federal income tax effect	0	24
Tax benefit from organizational restructuring	0	13
Credit for increasing research activities	4	0
Change in federal and state valuation allowance	(14)	6
Other, net	(2)	(3)
Income tax benefit (expense)	$ 22	$ (47)
Effective income tax rate	(16.00%)	(21.00%)