Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax Reconciliation, Change in Deferred Tax Assets Valuation Allowance	$ (6)	$ 33
Income tax benefit	$ 47	$ 292
Effective Income Tax Rate Reconciliation, Percent	21.40%	58.60%
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)
Tax benefit from organizational restructuring	13	0
Increase deferred tax liability for business activity changes	0	65
Unrecognized tax benefit	226		$ 239
Income Taxes Paid, Net	$ 39	$ 32