4. Securitization Trust Debt (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
"Securitization trust debt," and components of debt
Receivables Pledged at end of period	$ 1,792,208
Initial Principal	3,129,200
Outstanding Principal	$ 1,775,574	$ 1,598,496
CPS 2012-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2019
Receivables Pledged at end of period	$ 38,128
Initial Principal	141,500
Outstanding Principal	$ 37,281	50,125
Weighted Average Contractual Interest Rate end of period	3.07%
CPS 2012-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2019
Receivables Pledged at end of period	$ 43,231
Initial Principal	147,000
Outstanding Principal	$ 42,302	55,619
Weighted Average Contractual Interest Rate end of period	2.39%
CPS 2012-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2020
Receivables Pledged at end of period	$ 52,348
Initial Principal	160,000
Outstanding Principal	$ 51,078	67,833
Weighted Average Contractual Interest Rate end of period	2.02%
CPS 2013-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2020
Receivables Pledged at end of period	$ 76,253
Initial Principal	185,000
Outstanding Principal	$ 75,479	97,775
Weighted Average Contractual Interest Rate end of period	1.93%
CPS 2013-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2020
Receivables Pledged at end of period	$ 93,333
Initial Principal	205,000
Outstanding Principal	$ 92,611	118,692
Weighted Average Contractual Interest Rate end of period	2.42%
CPS 2013-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2020
Receivables Pledged at end of period	$ 107,454
Initial Principal	205,000
Outstanding Principal	$ 106,571	133,628
Weighted Average Contractual Interest Rate end of period	3.03%
CPS 2013-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2021
Receivables Pledged at end of period	$ 106,445
Initial Principal	183,000
Outstanding Principal	$ 105,390	132,150
Weighted Average Contractual Interest Rate end of period	2.67%
CPS 2014-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2021
Receivables Pledged at end of period	$ 117,005
Initial Principal	180,000
Outstanding Principal	$ 116,084	143,456
Weighted Average Contractual Interest Rate end of period	2.24%
CPS 2015-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2022
Receivables Pledged at end of period	$ 232,950
Initial Principal	245,000
Outstanding Principal	$ 230,019	0
Weighted Average Contractual Interest Rate end of period	2.36%
CPS 2011-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	April 2018
Receivables Pledged at end of period	$ 0
Initial Principal	100,364
Outstanding Principal	$ 0	8,457
Weighted Average Contractual Interest Rate end of period	0.00%
CPS 2011-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2018
Receivables Pledged at end of period	$ 16,276
Initial Principal	109,936
Outstanding Principal	$ 15,805	22,985
Weighted Average Contractual Interest Rate end of period	4.45%
CPS 2011-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2019
Receivables Pledged at end of period	$ 21,828
Initial Principal	119,400
Outstanding Principal	$ 21,794	30,601
Weighted Average Contractual Interest Rate end of period	4.90%
CPS 2012-A [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	June 2019
Receivables Pledged at end of period	$ 26,812
Initial Principal	155,000
Outstanding Principal	$ 25,311	35,923
Weighted Average Contractual Interest Rate end of period	3.26%
CPS 2014-B [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2021
Receivables Pledged at end of period	$ 150,384
Initial Principal	202,500
Outstanding Principal	$ 149,774	177,601
Weighted Average Contractual Interest Rate end of period	2.01%
CPS 2014-C [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	December 2021
Receivables Pledged at end of period	$ 224,185
Initial Principal	273,000
Outstanding Principal	$ 222,119	256,151
Weighted Average Contractual Interest Rate end of period	2.22%
CPS 2014-D [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	March 2022
Receivables Pledged at end of period	$ 237,229
Initial Principal	267,500
Outstanding Principal	$ 233,956	267,500
Weighted Average Contractual Interest Rate end of period	2.46%
CPS 2015-B(3) [Member]
"Securitization trust debt," and components of debt
Final Scheduled Payment Date	September 2022
Receivables Pledged at end of period	$ 248,347
Initial Principal	250,000
Outstanding Principal	$ 250,000	$ 0
Weighted Average Contractual Interest Rate end of period	2.47%