3. Securitization Trust Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 2,131,868
Initial Principal	4,196,295
Outstanding Principal	$ 2,095,252	$ 2,094,253
CPS 2012-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2019
Receivables Pledged at end of period	$ 0
Initial Principal	147,000
Outstanding Principal	$ 0	14,421
Weighted Average Contractual Interest Rate	0.00%
CPS 2012-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2020
Receivables Pledged at end of period	$ 15,920
Initial Principal	160,000
Outstanding Principal	$ 14,443	17,865
Weighted Average Contractual Interest Rate	1.88%
CPS 2013-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2020
Receivables Pledged at end of period	$ 25,391
Initial Principal	185,000
Outstanding Principal	$ 23,845	28,661
Weighted Average Contractual Interest Rate	1.71%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 34,073
Initial Principal	205,000
Outstanding Principal	$ 31,760	37,570
Weighted Average Contractual Interest Rate	2.18%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 41,253
Initial Principal	205,000
Outstanding Principal	$ 40,751	46,830
Weighted Average Contractual Interest Rate	4.89%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 41,847
Initial Principal	183,000
Outstanding Principal	$ 40,236	46,345
Weighted Average Contractual Interest Rate	4.28%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 49,623
Initial Principal	180,000
Outstanding Principal	$ 47,812	54,988
Weighted Average Contractual Interest Rate	3.63%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 67,362
Initial Principal	202,500
Outstanding Principal	$ 66,722	75,140
Weighted Average Contractual Interest Rate	3.12%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 104,233
Initial Principal	273,000
Outstanding Principal	$ 103,419	116,280
Weighted Average Contractual Interest Rate	3.27%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 113,895
Initial Principal	267,500
Outstanding Principal	$ 113,507	127,307
Weighted Average Contractual Interest Rate	3.48%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 122,123
Initial Principal	245,000
Outstanding Principal	$ 122,207	134,466
Weighted Average Contractual Interest Rate	3.10%
CPS 2015-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 140,289
Initial Principal	250,000
Outstanding Principal	$ 139,941	153,893
Weighted Average Contractual Interest Rate	3.11%
CPS 2015-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2022
Receivables Pledged at end of period	$ 190,437
Initial Principal	300,000
Outstanding Principal	$ 190,367	207,636
Weighted Average Contractual Interest Rate	3.54%
CPS 2016-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2023
Receivables Pledged at end of period	$ 242,144
Initial Principal	329,460
Outstanding Principal	$ 240,391	262,260
Weighted Average Contractual Interest Rate	3.90%
CPS 2016-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2023
Receivables Pledged at end of period	$ 271,079
Initial Principal	332,690
Outstanding Principal	$ 264,265	284,752
Weighted Average Contractual Interest Rate	3.92%
CPS 2016-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2023
Receivables Pledged at end of period	$ 274,132
Initial Principal	318,500
Outstanding Principal	$ 267,108	285,618
Weighted Average Contractual Interest Rate	3.48%
CPS 2016-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2023
Receivables Pledged at end of period	$ 194,366
Initial Principal	206,325
Outstanding Principal	$ 188,640	200,221
Weighted Average Contractual Interest Rate	2.80%
CPS 2017-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	April 2024
Receivables Pledged at end of period	$ 203,701
Initial Principal	206,320
Outstanding Principal	$ 199,838	$ 0
Weighted Average Contractual Interest Rate	3.00%