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Description of Business (Percentage of Consumer Loans Assigned with FICO Score of Less Than 650 or No FICO Score) (Details) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Percentage Of Contracts With FICO Score Lower Than 650 Or No FICO Score [Member] |  |  |
Financing Receivable, Credit Quality Indicator [Line Items] |  |  |
Financing Receivable, Credit Quality, Additional Information | 95.90% | 95.60% | 95.60%

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- Definition Financing Receivable, Credit Quality, Impaired Credit Scores, Percentage + References No definition available. + Details
Name: | cacc_FinancingReceivableCreditQualityImpairedCreditScoresPercentage
Namespace Prefix: | cacc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_CreditScoreFicoAxis=cacc_PercentageOfContractsWithFICOScoreLowerThan650OrNoFICOScoreMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |