Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetReportedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	cacc_LoansAxis=cacc_LoanPoolPerformanceMeetsOrExceedsInitialEstimatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cacc_LoansAxis=cacc_LoanPoolPerformanceLessThanInitialEstimatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=cacc_DealerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=cacc_PurchasedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans Receivable (Schedule of Consumer Loans Performance) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 4,637.7	$ 4,474.0	$ 4,207.0	$ 3,812.0	$ 3,626.2	$ 3,345.1
Allowance for credit losses	(354.1)	(338.0)	(320.4)	(276.9)	(262.8)	(243.6)
Loans receivable, net	4,283.6		3,886.6
Loan Pool Performance Meets Or Exceeds Initial Estimates [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,789.6		1,708.0
Loans receivable, net	1,789.6		1,708.0
Loan Pool Performance Less Than Initial Estimates [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,848.1		2,499.0
Allowance for credit losses	(354.1)		(320.4)
Loans receivable, net	2,494.0		2,178.6
Dealer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	3,385.3	3,326.5	3,209.0	3,049.2	2,985.7	2,823.4
Allowance for credit losses	(336.9)	(323.8)	(309.3)	(268.2)	(254.0)	(235.1)
Loans receivable, net	3,048.4		2,899.7
Dealer Loans [Member] \| Loan Pool Performance Meets Or Exceeds Initial Estimates [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,104.6		1,002.2
Loans receivable, net	1,104.6		1,002.2
Dealer Loans [Member] \| Loan Pool Performance Less Than Initial Estimates [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,280.7		2,206.8
Allowance for credit losses	(336.9)		(309.3)
Loans receivable, net	1,943.8		1,897.5
Purchased Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,252.4	1,147.5	998.0	762.8	640.5	521.7
Allowance for credit losses	(17.2)	$ (14.2)	(11.1)	$ (8.7)	$ (8.8)	$ (8.5)
Loans receivable, net	1,235.2		986.9
Purchased Loans [Member] \| Loan Pool Performance Meets Or Exceeds Initial Estimates [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	685.0		705.8
Loans receivable, net	685.0		705.8
Purchased Loans [Member] \| Loan Pool Performance Less Than Initial Estimates [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	567.4		292.2
Allowance for credit losses	(17.2)		(11.1)
Loans receivable, net	$ 550.2		$ 281.1