Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at the federal statutory rate	$ (46)	$ (174)
State income taxes, net of federal income tax effect	(15)	(22)
Cumulative effect of changes in corporate state income tax laws that were recognized within State income taxes, net of federal income tax effect	24	3
Increase deferred tax liability for business activity changes	0	(65)
Credit for increasing research activities	0	4
Tax benefit from organizational restructuring	13	0
Change in federal and state valuation allowance	6	(33)
Other, net	(3)	(5)
Income tax expense	$ (47)	$ (292)
Effective income tax rate	(21.40%)	(58.60%)