5. Securitization Trust Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 1,757,096
Initial Principal	3,429,200
Outstanding Principal	$ 1,731,598	$ 1,598,496
CPS 2011-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	April 2018
Receivables Pledged at end of period	$ 0
Initial Principal	100,364
Outstanding Principal	$ 0	8,457
Weighted Average Contractual Interest Rate	0.00%
CPS 2011-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2018
Receivables Pledged at end of period	$ 10,469
Initial Principal	109,936
Outstanding Principal	$ 10,023	22,985
Weighted Average Contractual Interest Rate	4.40%
CPS 2011-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2019
Receivables Pledged at end of period	$ 14,756
Initial Principal	119,400
Outstanding Principal	$ 14,785	30,601
Weighted Average Contractual Interest Rate	4.91%
CPS 2012-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2019
Receivables Pledged at end of period	$ 18,273
Initial Principal	155,000
Outstanding Principal	$ 16,795	35,923
Weighted Average Contractual Interest Rate	3.15%
CPS 2012-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2019
Receivables Pledged at end of period	$ 27,463
Initial Principal	141,500
Outstanding Principal	$ 26,758	50,125
Weighted Average Contractual Interest Rate	3.07%
CPS 2012-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2019
Receivables Pledged at end of period	$ 31,556
Initial Principal	147,000
Outstanding Principal	$ 30,653	55,619
Weighted Average Contractual Interest Rate	2.34%
CPS 2012-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2020
Receivables Pledged at end of period	$ 38,886
Initial Principal	160,000
Outstanding Principal	$ 37,464	67,833
Weighted Average Contractual Interest Rate	1.92%
CPS 2013-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2020
Receivables Pledged at end of period	$ 58,082
Initial Principal	185,000
Outstanding Principal	$ 56,583	97,775
Weighted Average Contractual Interest Rate	1.85%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 72,081
Initial Principal	205,000
Outstanding Principal	$ 70,332	118,692
Weighted Average Contractual Interest Rate	2.36%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 83,811
Initial Principal	205,000
Outstanding Principal	$ 82,851	113,628
Weighted Average Contractual Interest Rate	3.43%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 83,263
Initial Principal	183,000
Outstanding Principal	$ 82,337	132,150
Weighted Average Contractual Interest Rate	2.99%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 93,776
Initial Principal	180,000
Outstanding Principal	$ 92,571	143,456
Weighted Average Contractual Interest Rate	2.51%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 121,772
Initial Principal	202,500
Outstanding Principal	$ 121,515	177,601
Weighted Average Contractual Interest Rate	2.21%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 184,739
Initial Principal	273,000
Outstanding Principal	$ 183,802	256,151
Weighted Average Contractual Interest Rate	2.41%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 200,611
Initial Principal	267,500
Outstanding Principal	$ 198,533	267,500
Weighted Average Contractual Interest Rate	2.63%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 203,819
Initial Principal	245,000
Outstanding Principal	$ 201,527	0
Weighted Average Contractual Interest Rate	2.48%
CPS 2015-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 226,054
Initial Principal	250,000
Outstanding Principal	$ 221,587	0
Weighted Average Contractual Interest Rate	2.57%
CPS 2015-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2022
Receivables Pledged at end of period	$ 287,685
Initial Principal	300,000
Outstanding Principal	$ 283,482	$ 0
Weighted Average Contractual Interest Rate	2.96%