Income Tax Provision (Deferred Tax Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	$ 6,236	$ 5,412
Tax Liabilities
Lease transactions	6,509	6,379
State Taxes	446	429
Mark-to-market of investments in marketable securities	210	48
Other	295	296
Deferred tax liabilities	7,460	7,152
Tax Assets
Provsion for credit and residual value losses	347	369
Deferred costs and fees	167	155
Net operating loss and tax credit carry forwards	679	1,168
Other	47	52
Deferred tax assets	1,240	1,744
Valuation Allowance	(16)	(4)
Net deferred tax asset	1,224	1,740
Net deferred income tax liability	6,236	5,412
Federal [Member]
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	5,763	5,009
Tax Assets
Net deferred income tax liability	5,763	5,009
State [Member]
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	475	406
Tax Assets
Net deferred income tax liability	475	406
Foreign [Member]
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	(2)	(3)
Tax Assets
Net deferred income tax liability	$ (2)	$ (3)