Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2014
Net financing revenues:
Manufacturer's subvention support and other revenues	$ 315	$ 251	$ 884	$ 733
Credit support fees incurred	(23)	(21)	(66)	(61)
Interest expense on loans payable to affiliates	(1)	(1)	(2)	(2)
Insurance earned premiums and contract revenues:
Affiliate insurance premiums and contract revenues	32	33	97	99
Investments and other income, net:
Interest earned on notes receivable from affiliates	1	1	3	4
Expenses:
Shared services charges and other expenses	16	15	48	45
Employee benefits expense	5	9	18	27
Finance receivables, net
Accounts receivable from affiliates	85		85		74
Direct finance receivables from affiliates	6		6		6
Notes receivable under home loan programs	12		12		15
Deferred retail origination costs paid to affiliates	1		1		1
Deferred retail subvention income from affiliates	(819)		(819)		(768)
Investments in operating leases, net
Leases to affiliates	7		7		7
Deferred lease subvention income from affiliates	(954)		(954)		(806)
Other assets
Notes receivable from alliliates	821		821		1,172
Other receivables from affiliates	5		5		2
Subvention support receivable from affiliates	119		119		159
Other liabilities
Unearned affiliates insurance premiums and contract revenues	252		252		244
Accounts payable to affiliates	116		116		216
Notes payable to affiliate	28		28		22
Shareholder's Equity:
Dividends paid	435				665
Stock-based compensation	$ 2				$ 2