Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Mar. 31, 2014
Net financing revenues:
Manufacturer's subvention support and other revenues	$ 296	$ 250	$ 569	$ 482
Credit support fees incurred	(22)	(20)	(43)	(40)
Interest expense on loans payable to affiliates	0	(1)	(1)	(1)
Insurance earned premiums and contract revenues:
Affiliate insurance premiums and contract revenues	33	33	65	66
Investments and other income, net:
Interest earned on notes receivable from affiliates	1	2	2	3
Expenses:
Shared services charges and other expenses	16	16	32	30
Employee benefits expense	7	9	13	18
Finance receivables, net
Accounts receivable from affiliates	80		80		74
Direct finance receivables from affiliates	6		6		6
Notes receivable under home loan programs	14		14		15
Deferred retail origination costs paid to affiliates	1		1		1
Deferred retail subvention income from affiliates	(839)		(839)		(768)
Investments in marketable securities [Abstract]
Investments in affiliates' commercial paper	46		46		0
Investments in operating leases, net
Leases to affiliates	7		7		7
Deferred lease subvention income from affiliates	(966)		(966)		(806)
Other assets
Notes receivable from alliliates	336		336		1,172
Other receivables from affiliates	2		2		2
Subvention support receivable from affiliates	159		159		159
Other liabilities
Unearned affiliates insurance premiums and contract revenues	247		247		244
Accounts payable to affiliates	149		149		216
Notes payable to affiliate	33		33		22
Shareholder's Equity:
Dividends paid	435				665
Stock-based compensation	$ 2				$ 2