Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012
Net financing revenues:
Manufacturer's subvention support and other revenues	$ 236	$ 243	$ 462	$ 486
Credit support fees incurred	(27)	(8)	(35)	(16)
Foreign exchange gain (loss) on loans payable to affiliates	0	(56)	(39)	(94)
Interest expense on loans payable to affiliates	(1)	(12)	(4)	(23)
Insurance earned premiums and contract revenues:
Affiliate insurance premiums and contract revenues	43	55	92	109
Investments and other income, net:
Interest earned on notes receivable from affiliates	2	1	3	1
Expenses:
Shared services charges and other expenses	16	17	32	34
Employee benefits expense	8	10	15	20
Investment in marketable securities
Investments in marketable securities	5		5		0
Finance receivables, net
Accounts receivable from affiliates	16		16		17
Direct finance receivables from affiliates	3		3		4
Notes receivable under home loan programs	20		20		19
Deferred retail subvention income from affiliates	(667)		(667)		(598)
Investments in operating leases, net
Leases to affiliates	4		4		4
Deferred lease subvention income from affiliates	(600)		(600)		(592)
Other assets
Notes receivable from alliliates	887		887		1,052
Other receivables from affiliates	5		5		8
Subvention support receivable from affiliates	103		103		65
Debt:
Loans payable to affiliate	0		0		2,201
Other liabilities
Unearned affiliates insurance premiums and contract revenues	241		241		273
Accounts payable to affiliates	134		134		58
Notes payable to affiliate	61		61		61
Shareholder's Equity:
Dividends paid	744		744		741
Stock based compensation	$ 2				$ 2