Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	31.50%
Income tax benefit (expense) at the federal statutory rate	$ (95)	$ 270	$ 649
State income taxes, net of federal income tax effect	(43)	24	38
State law changes, net of federal income tax effect	9	4	20
Reduction (increase) in liability for unrecognized tax benefits	(29)	(14)	(4)
Effective Income Tax Rate Reconciliation, Tax Contingency, State and Local, Amount	(89)	0	0
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Percent	15	15	14
Tax (expense) benefit from organizational restructuring	0	(118)	90
Change in valuation allowance	224	(615)	(939)
Effective Income Tax Rate Reconciliation, Tax benefit from the Tax Act	7,088	0	0
Other, net	(6)	(1)	(9)
Total income tax (expense) benefit	$ 7,074	$ (435)	$ (141)
Effective income tax rate	2334.70%	56.40%	(7.60%)