5. Securitization Trust Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 2,141,613
Initial Principal	4,515,811
Outstanding Principal	$ 2,094,253	$ 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	$ 0
Initial Principal	119,400
Outstanding Principal	$ 0	14,785
Weighted Average Contractual Interest Rate
CPS 2012-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2019
Receivables Pledged at end of period	$ 0
Initial Principal	155,000
Outstanding Principal	$ 0	16,795
Weighted Average Contractual Interest Rate
CPS 2012-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2019
Receivables Pledged at end of period	$ 0
Initial Principal	141,500
Outstanding Principal	$ 0	26,758
Weighted Average Contractual Interest Rate
CPS 2012-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2019
Receivables Pledged at end of period	$ 15,131
Initial Principal	147,000
Outstanding Principal	$ 14,421	30,653
Weighted Average Contractual Interest Rate	2.46%
CPS 2012-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2020
Receivables Pledged at end of period	$ 19,232
Initial Principal	160,000
Outstanding Principal	$ 17,865	37,464
Weighted Average Contractual Interest Rate	1.86%
CPS 2013-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2020
Receivables Pledged at end of period	$ 30,258
Initial Principal	185,000
Outstanding Principal	$ 28,661	56,583
Weighted Average Contractual Interest Rate	1.73%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 40,395
Initial Principal	205,000
Outstanding Principal	$ 37,570	70,332
Weighted Average Contractual Interest Rate	2.19%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 48,080
Initial Principal	205,000
Outstanding Principal	$ 46,830	82,851
Weighted Average Contractual Interest Rate	4.65%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 48,487
Initial Principal	183,000
Outstanding Principal	$ 46,345	82,337
Weighted Average Contractual Interest Rate	4.10%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 57,171
Initial Principal	180,000
Outstanding Principal	$ 54,988	92,571
Weighted Average Contractual Interest Rate	3.39%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 76,158
Initial Principal	202,500
Outstanding Principal	$ 75,140	121,515
Weighted Average Contractual Interest Rate	2.90%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 117,071
Initial Principal	273,000
Outstanding Principal	$ 116,280	183,802
Weighted Average Contractual Interest Rate	3.05%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 127,721
Initial Principal	267,500
Outstanding Principal	$ 127,307	198,533
Weighted Average Contractual Interest Rate	3.27%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 136,010
Initial Principal	245,000
Outstanding Principal	$ 134,466	201,527
Weighted Average Contractual Interest Rate	2.96%
CPS 2015-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 155,303
Initial Principal	250,000
Outstanding Principal	$ 153,893	221,587
Weighted Average Contractual Interest Rate	2.97%
CPS 2015-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2022
Receivables Pledged at end of period	$ 210,026
Initial Principal	300,000
Outstanding Principal	$ 283,482	283,482
Weighted Average Contractual Interest Rate	3.39%
CPS 2016-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2023
Receivables Pledged at end of period	$ 265,496
Initial Principal	329,460
Outstanding Principal	$ 262,260	0
Weighted Average Contractual Interest Rate	3.15%
CPS 2016-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2023
Receivables Pledged at end of period	$ 294,463
Initial Principal	332,690
Outstanding Principal	$ 284,752	0
Weighted Average Contractual Interest Rate	3.79%
CPS 2016-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2023
Receivables Pledged at end of period	$ 296,179
Initial Principal	318,500
Outstanding Principal	$ 285,618	0
Weighted Average Contractual Interest Rate	3.36%
CPS 2016-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2023
Receivables Pledged at end of period	$ 204,432
Initial Principal	206,325
Outstanding Principal	$ 200,221	$ 0
Weighted Average Contractual Interest Rate	2.73%