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Description of Business (Percentage of Consumer Loans Assigned with FICO Score of Less Than 650 or No FICO Score)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Percentage Of Contracts With FICO Score Lower Than 650 Or No FICO Score [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage Of Contracts	92.70%	95.60%	93.60%	96.10%

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- Definition

Percentage Of Contracts

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No definition available.

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- Definition

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.

+ References

No definition available.

+ Details

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

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- Details

Name:	us-gaap_CreditScoreFicoAxis=cacc_FICOScoreLowerThan650OrNoFICOScoreMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: