Income Taxes (Effective Income Tax Rate) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	21.00%	31.50%	35.00%
Income tax benefit (expense) at the federal statutory rate	$ 415	$ (95)	$ 270
State income taxes, net of federal income tax effect	(15)	(43)	24
State law changes, net of federal income tax effect	85	9	4
Reduction (increase) in liability for unrecognized tax benefits	(8)	(29)	(14)
Effective Income Tax Rate Reconciliation, Tax Contingency, State and Local, Amount	0	(89)	0
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Percent	17	15	15
Tax (expense) benefit from organizational restructuring	(13)	0	(118)
Change in valuation allowance	(8)	224	(615)
Effective Income Tax Rate Reconciliation, Tax benefit from the Tax Act	0	7,088	0
Non-deductible penalties	(29)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount	(408)
Other, net	(1)	(6)	(1)
Total income tax (expense) benefit	$ 35	$ 7,074	$ (435)
Effective income tax rate	(1.80%)	2334.70%	(56.40%)