Summary of Significant Accounting Policies - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2015 segment	Dec. 31, 2014
Summary of Significant Accounting Policies [Line Items]
Decrease to net cash provided by operating activities	$ 79,000,000
Increase to net cash used in investing activities	79,000,000
Number of operating segments	2
Related party receivables	437,000,000	384,000,000
Intercompany Receivable - Commercial Lending	164,000,000	176,000,000
Intercompany Receivables - Wholesale financing	0	289,000,000
Related party payables	450,000,000	433,000,000
Related party payable settlement period	30 days
Related party taxes payable	636,000,000	636,000,000
Earning assets leverage ratio	6.9
Parent Company [Member] | UNITED STATES
Summary of Significant Accounting Policies [Line Items]
Related party tax expense deferral period	4 years
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Earning assets leverage ratio	8.0
Junior Subordinated Revolving Credit Facility [Member]
Summary of Significant Accounting Policies [Line Items]
Line of Credit Facilities - GM Related Party Facility	1,000,000,000
General Motors [Member]
Summary of Significant Accounting Policies [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 4,000,000,000