Summary of Significant Accounting Policies - Narrative (Details)	9 Months Ended
	Sep. 30, 2015 USD ($) segment	Dec. 31, 2014 USD ($)
Summary of Significant Accounting Policies [Line Items]
Number of operating segments | segment	2
Related party receivables	$ 589,000,000	$ 384,000,000
Intercompany Receivable - Commercial Lending	186,000,000	176,000,000
Intercompany Receivables - Wholesale financing	0	289,000,000
Related party payables	$ 527,000,000	433,000,000
Related party payable settlement period	30 days
Related party taxes payable	$ 649,000,000	$ 636,000,000
Earning assets leverage ratio	8.4
Parent Company [Member] | United States [Member]
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	9.5
Junior Subordinated Revolving Credit Facility [Member]
Summary of Significant Accounting Policies [Line Items]
Line of Credit Facilities - GM Related Party Facility	$ 1,000,000,000.0
General Motors [Member]
Summary of Significant Accounting Policies [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 4,000,000,000.0