VIEs - Narrative (Details) $ in Billions	3 Months Ended				6 Months Ended
	Jun. 30, 2015 USD ($)	Dec. 31, 2012 USD ($)	Mar. 31, 2012 USD ($)	Sep. 30, 2011 USD ($)	Jun. 30, 2015 trust
Variable Interest Entity [Line Items]
Number of securitization trusts acquired | trust					2
Total of automobile loans transferred in securitization transactions | $	$ 0.8	$ 1.0	$ 1.3	$ 1.0