Debt Obligations - Securitization Debt (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	$ 1,108,796		$ 1,049,478
Repurchase Option Percentage	10.00%
Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	720,967		677,118
Gross Receivables Pledged	897,636		873,231
Cash Reserve	31,200		26,700
2010-1 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	11,850		23,036
Gross Receivables Pledged	30,375		55,525
Cash Reserve	4,500		4,500
Interest Rate	3.60%	[1]	3.60%	[1]
2011-1 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	38,399		60,335
Gross Receivables Pledged	53,320		84,198
Cash Reserve	4,200		4,200
Interest Rate	3.00%	[1]	3.00%	[1]
2011-2 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	57,078		84,977
Gross Receivables Pledged	69,076		103,779
Cash Reserve	4,500		4,500
Interest Rate	2.90%	[1]	2.90%	[1]
2011-3 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	89,730		130,347
Gross Receivables Pledged	109,880		159,068
Cash Reserve	4,500		4,500
Interest Rate	3.90%	[1]	3.90%	[1]
2012-1 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	125,640		170,198
Gross Receivables Pledged	161,453		219,252
Cash Reserve	4,500		4,500
Interest Rate	3.50%	[1]	3.50%	[1]
2012-2 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	161,280		208,225
Gross Receivables Pledged	199,302		251,409
Cash Reserve	4,500		4,500
Interest Rate	2.90%	[1]	2.90%	[1]
2013-1 [Member] | Securitization Debt [Member]
Assets that Continue to be Recognized, Securitized or Asset-backed Financing Arrangement Assets and any Other Financial Assets Managed Together [Line Items]
Debt Balance	236,990
Gross Receivables Pledged	274,230
Cash Reserve	$ 4,500
Interest Rate	2.70%	[1]

[1] These rates represent the original duration weighted average rates of the outstanding asset-backed securities.