Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	35.00%
Income tax benefit (expense) at the federal statutory rate	$ 270	$ 649	$ 1,372
State income taxes, net of federal income tax effect	24	38	124
State law changes, net of federal income tax effect	4	20	4
Reduction (increase) in liability for unrecognized tax benefits	(14)	(4)	1
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Percent	15	14	0
Tax (expense) benefit from organizational restructuring	(118)	90	0
Change in valuation allowance	(615)	(939)	(911)
Other, net	(1)	(9)	(16)
Total income tax (expense) benefit	$ (435)	$ (141)	$ 574
Effective income tax rate	56.40%	7.60%	14.60%