3. Securitization Trust Debt (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 2,142,835
Initial Principal	4,421,465
Outstanding Principal	$ 2,094,994	$ 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	$ 0
Initial Principal	160,000
Outstanding Principal	$ 0	17,865
Weighted Average Contractual Interest Rate	0.00%
CPS 2013-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2020
Receivables Pledged at end of period	$ 21,277
Initial Principal	185,000
Outstanding Principal	$ 19,482	28,661
Weighted Average Contractual Interest Rate	1.72%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 29,141
Initial Principal	205,000
Outstanding Principal	$ 26,540	37,570
Weighted Average Contractual Interest Rate	2.15%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 35,387
Initial Principal	205,000
Outstanding Principal	$ 34,653	46,830
Weighted Average Contractual Interest Rate	5.23%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 35,962
Initial Principal	183,000
Outstanding Principal	$ 34,333	46,345
Weighted Average Contractual Interest Rate	4.52%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 43,308
Initial Principal	180,000
Outstanding Principal	$ 41,114	54,988
Weighted Average Contractual Interest Rate	3.83%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 59,344
Initial Principal	202,500
Outstanding Principal	$ 58,167	75,140
Weighted Average Contractual Interest Rate	3.35%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 93,080
Initial Principal	273,000
Outstanding Principal	$ 91,285	116,280
Weighted Average Contractual Interest Rate	3.53%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 101,294
Initial Principal	267,500
Outstanding Principal	$ 100,325	127,307
Weighted Average Contractual Interest Rate	3.74%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 109,649
Initial Principal	245,000
Outstanding Principal	$ 108,821	134,466
Weighted Average Contractual Interest Rate	3.29%
CPS 2015-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 126,623
Initial Principal	250,000
Outstanding Principal	$ 125,584	153,893
Weighted Average Contractual Interest Rate	3.27%
CPS 2015-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2022
Receivables Pledged at end of period	$ 172,332
Initial Principal	300,000
Outstanding Principal	$ 171,144	207,636
Weighted Average Contractual Interest Rate	3.74%
CPS 2016-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2023
Receivables Pledged at end of period	$ 220,103
Initial Principal	329,460
Outstanding Principal	$ 218,351	262,260
Weighted Average Contractual Interest Rate	4.07%
CPS 2016-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2023
Receivables Pledged at end of period	$ 247,067
Initial Principal	332,690
Outstanding Principal	$ 240,209	284,752
Weighted Average Contractual Interest Rate	4.11%
CPS 2016-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2023
Receivables Pledged at end of period	$ 250,358
Initial Principal	318,500
Outstanding Principal	$ 244,183	285,618
Weighted Average Contractual Interest Rate	3.65%
CPS 2016-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2023
Receivables Pledged at end of period	$ 180,742
Initial Principal	206,325
Outstanding Principal	$ 175,822	200,221
Weighted Average Contractual Interest Rate	2.90%
CPS 2017-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	April 2024
Receivables Pledged at end of period	$ 193,575
Initial Principal	206,320
Outstanding Principal	$ 187,641	0
Weighted Average Contractual Interest Rate	3.08%
CPS 2017-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2024
Receivables Pledged at end of period	$ 223,593
Initial Principal	225,170
Outstanding Principal	$ 217,340	$ 0
Weighted Average Contractual Interest Rate	2.74%