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Loans Receivable (Narrative) (Details) | 9 Months Ended
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Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Age of Consumer Loans contractual payments due with no forecasted future net cash flows | 120 months

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- Definition Period After Consumer Loans With No Forecasted Future Net Cash Flows + References No definition available. + Details
Name: | cacc_PeriodAfterConsumerLoansWithNoForecastedFutureNetCashFlows
Namespace Prefix: | cacc_
Data Type: | xbrli:durationItemType
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration