3. Securitization Trust Debt (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 2,176,329
Initial Principal	4,646,290
Outstanding Principal	$ 2,116,399	$ 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
Initial Principal	147,000
Outstanding Principal		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
Initial Principal	160,000
Outstanding Principal		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	$ 18,270
Initial Principal	185,000
Outstanding Principal	$ 16,015	28,661
Weighted Average Contractual Interest Rate	1.70%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 25,007
Initial Principal	205,000
Outstanding Principal	$ 22,374	37,570
Weighted Average Contractual Interest Rate	2.13%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 30,755
Initial Principal	205,000
Outstanding Principal	$ 29,939	46,830
Weighted Average Contractual Interest Rate	5.58%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 31,306
Initial Principal	183,000
Outstanding Principal	$ 29,557	46,345
Weighted Average Contractual Interest Rate	4.77%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 37,687
Initial Principal	180,000
Outstanding Principal	$ 35,414	54,988
Weighted Average Contractual Interest Rate	4.06%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 52,509
Initial Principal	202,500
Outstanding Principal	$ 51,099	75,140
Weighted Average Contractual Interest Rate	3.49%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 82,954
Initial Principal	273,000
Outstanding Principal	$ 81,051	116,280
Weighted Average Contractual Interest Rate	3.72%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 90,853
Initial Principal	267,500
Outstanding Principal	$ 89,368	127,307
Weighted Average Contractual Interest Rate	4.02%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 98,727
Initial Principal	245,000
Outstanding Principal	$ 97,395	134,466
Weighted Average Contractual Interest Rate	3.50%
CPS 2015-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 115,193
Initial Principal	250,000
Outstanding Principal	$ 114,161	153,893
Weighted Average Contractual Interest Rate	3.43%
CPS 2015-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2022
Receivables Pledged at end of period	$ 156,908
Initial Principal	300,000
Outstanding Principal	$ 155,261	207,636
Weighted Average Contractual Interest Rate	3.94%
CPS 2016-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2023
Receivables Pledged at end of period	$ 200,682
Initial Principal	329,460
Outstanding Principal	$ 198,375	262,260
Weighted Average Contractual Interest Rate	4.25%
CPS 2016-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2023
Receivables Pledged at end of period	$ 226,642
Initial Principal	332,690
Outstanding Principal	$ 220,324	284,752
Weighted Average Contractual Interest Rate	4.29%
CPS 2016-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2023
Receivables Pledged at end of period	$ 228,946
Initial Principal	318,500
Outstanding Principal	$ 222,544	285,618
Weighted Average Contractual Interest Rate	3.85%
CPS 2016-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2023
Receivables Pledged at end of period	$ 166,557
Initial Principal	206,325
Outstanding Principal	$ 162,871	200,221
Weighted Average Contractual Interest Rate	3.01%
CPS 2017-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	April 2024
Receivables Pledged at end of period	$ 179,926
Initial Principal	206,320
Outstanding Principal	$ 174,942	0
Weighted Average Contractual Interest Rate	3.18%
CPS 2017-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2024
Receivables Pledged at end of period	$ 212,943
Initial Principal	225,170
Outstanding Principal	$ 201,190	0
Weighted Average Contractual Interest Rate	2.81%
CPS 2017-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2024
Receivables Pledged at end of period	$ 220,464
Initial Principal	224,825
Outstanding Principal	$ 214,519	$ 0
Weighted Average Contractual Interest Rate	2.74%