Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at the federal statutory rate	$ (174)	$ 86
State income taxes, net of federal income tax effect	(22)	3
Cumulative effect of changes in corporate state income tax laws that were recognized within State income taxes, net of federal income tax effect	3	0
Increase deferred tax liability for business activity changes	(65)	0
Credit for increasing research activities	$ 4	0
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate	35.00%
Change in federal and state valuation allowance	$ (33)	(142)
Other, net	(5)	(3)
Income tax expense	$ (292)	$ (56)
Effective income tax rate	(58.60%)	22.80%