Fair Value of Financial Instruments (Details) (USD $)	Sep. 30, 2013		Jun. 30, 2013	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Dealer Finance Receivables - GO, net	$ 105,796,000		$ 92,278,000	$ 40,956,000		$ 23,656,000	$ 11,633,000	$ 24,000
Portfolio Term Financings	1,177,818,000			1,049,478,000
Portfolio Warehouse Facilities	54,000,000			57,200,000
Senior Secured Notes Payable	253,543,000			193,320,000
Other Secured Notes Payable	148,965,000			117,281,000
Securitization Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	822,677,000			677,118,000
Portfolio Term Residual Financing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	100,000,000			25,000,000
Revolving Inventory Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	121,321,000			91,320,000
Mortgage Note Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	12,288,000			12,454,000
Real Estate Credit Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	14,014,000			11,733,000
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance Receivables, net	1,574,760,000	[1]		1,370,800,000	[1]
Dealer Finance Receivables - GO, net	105,796,000			40,956,000
Portfolio Warehouse Facilities	54,000,000			57,200,000
Senior Secured Notes Payable	253,543,000			198,320,000
Carrying Value [Member] | Securitization Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	822,677,000			677,118,000
Carrying Value [Member] | Portfolio Term Residual Financing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	100,000,000			25,000,000
Carrying Value [Member] | Bank Term Financings [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	255,141,000			347,360,000
Carrying Value [Member] | Revolving Inventory Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	121,321,000			91,320,000
Carrying Value [Member] | Mortgage Note Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	12,288,000			12,454,000
Carrying Value [Member] | Real Estate Credit Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	14,014,000			11,733,000
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance Receivables, net	0	[1]		0	[1]
Dealer Finance Receivables - GO, net	0			0
Portfolio Warehouse Facilities	0			0
Senior Secured Notes Payable	281,433,000			220,135,000
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] | Securitization Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	829,795,000			702,031,000
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] | Portfolio Term Residual Financing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	0			0
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] | Bank Term Financings [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	0			0
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] | Revolving Inventory Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] | Mortgage Note Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] | Real Estate Credit Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance Receivables, net	0	[1]		0	[1]
Dealer Finance Receivables - GO, net	0			0
Portfolio Warehouse Facilities	54,000,000			57,200,000
Senior Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Securitization Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	0			0
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Portfolio Term Residual Financing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	112,519,000			25,000,000
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Bank Term Financings [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	255,141,000			347,360,000
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Revolving Inventory Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	120,284,000			91,600,000
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Mortgage Note Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	11,051,000			11,200,000
Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] | Real Estate Credit Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	13,975,000			11,700,000
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance Receivables, net	1,648,065,000	[1]		1,441,026,000	[1]
Dealer Finance Receivables - GO, net	111,748,000			40,956,000
Portfolio Warehouse Facilities	0			0
Senior Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member] | Securitization Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	0			0
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member] | Portfolio Term Residual Financing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	0			0
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member] | Bank Term Financings [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Portfolio Term Financings	0			0
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member] | Revolving Inventory Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage Note Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	0			0
Fair Value [Member] | Fair Value, Inputs, Level 3 [Member] | Real Estate Credit Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Secured Notes Payable	$ 0			$ 0

[1] Represents finance receivable principal balances, plus accrued interest, less the allowance for credit losses.