3. Securitization Trust Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 1,892,943
Initial Principal	3,658,296
Outstanding Principal	$ 1,868,451	$ 1,731,598
CPS 2011-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2018
Receivables Pledged at end of period	$ 0
Initial Principal	109,936
Outstanding Principal	$ 0	10,023
Weighted Average Contractual Interest Rate
CPS 2011-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2019
Receivables Pledged at end of period	$ 11,958
Initial Principal	119,400
Outstanding Principal	$ 12,000	14,785
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	$ 14,785
Initial Principal	155,000
Outstanding Principal	$ 13,461	16,795
Weighted Average Contractual Interest Rate	3.09%
CPS 2012-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2019
Receivables Pledged at end of period	$ 22,682
Initial Principal	141,500
Outstanding Principal	$ 22,490	26,758
Weighted Average Contractual Interest Rate	3.06%
CPS 2012-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2019
Receivables Pledged at end of period	$ 26,217
Initial Principal	147,000
Outstanding Principal	$ 26,004	30,653
Weighted Average Contractual Interest Rate	2.36%
CPS 2012-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2020
Receivables Pledged at end of period	$ 33,015
Initial Principal	160,000
Outstanding Principal	$ 31,916	37,464
Weighted Average Contractual Interest Rate	1.90%
CPS 2013-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2020
Receivables Pledged at end of period	$ 50,001
Initial Principal	185,000
Outstanding Principal	$ 48,693	56,583
Weighted Average Contractual Interest Rate	1.80%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 62,166
Initial Principal	205,000
Outstanding Principal	$ 60,768	70,332
Weighted Average Contractual Interest Rate	2.33%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 73,202
Initial Principal	205,000
Outstanding Principal	$ 72,651	82,851
Weighted Average Contractual Interest Rate	3.68%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 72,948
Initial Principal	183,000
Outstanding Principal	$ 71,963	82,337
Weighted Average Contractual Interest Rate	3.19%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 82,550
Initial Principal	180,000
Outstanding Principal	$ 82,371	92,571
Weighted Average Contractual Interest Rate	2.67%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 108,109
Initial Principal	202,500
Outstanding Principal	$ 108,183	121,515
Weighted Average Contractual Interest Rate	2.35%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 165,468
Initial Principal	273,000
Outstanding Principal	$ 165,152	183,802
Weighted Average Contractual Interest Rate	2.54%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 178,281
Initial Principal	267,500
Outstanding Principal	$ 179,567	198,533
Weighted Average Contractual Interest Rate	2.75%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 184,209
Initial Principal	245,000
Outstanding Principal	$ 183,802	201,527
Weighted Average Contractual Interest Rate	2.57%
CPS 2015-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 207,940
Initial Principal	250,000
Outstanding Principal	$ 206,213	221,587
Weighted Average Contractual Interest Rate	2.64%
CPS 2015-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2022
Receivables Pledged at end of period	$ 271,082
Initial Principal	300,000
Outstanding Principal	$ 266,817	$ 283,482
Weighted Average Contractual Interest Rate	3.03%
CPS 2016-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2023
Receivables Pledged at end of period	$ 328,330
Initial Principal	329,460
Outstanding Principal	$ 316,400
Weighted Average Contractual Interest Rate	3.32%