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Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2017 USD ($) portfolio_segment segment | Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |
Number of reportable segments | segment | 1 |
Uninsured cash and cash equivalents | $ 4.6 | $ 14.3
Number of portfolio segments | portfolio_segment | 2 |
Period after Consumer Loan assignment that Dealer Loans written off | 120 months |
Period after month of purchase that Purchased Loans written off | 120 months |
Restricted Cash and Cash Equivalents [Member] |  |
Summary Of Significant Accounting Policies [Line Items] |  |
Uninsured cash and cash equivalents | $ 273.1 | $ 224.1

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- Definition Number of Portfolio Segments + References No definition available. + Details
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Namespace Prefix: | cacc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Period After Determination Dealer Loan Write Off + References No definition available. + Details
Name: | cacc_PeriodAfterDeterminationDealerLoanWriteOff
Namespace Prefix: | cacc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Period After Determination Purchased Loan Write Off + References No definition available. + Details
Name: | cacc_PeriodAfterDeterminationPurchasedLoanWriteOff
Namespace Prefix: | cacc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Summary Of Significant Accounting Policies [Line Items] + References No definition available. + Details
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Data Type: | xbrli:stringItemType
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- Definition The amount of cash as of the balance sheet date that is not insured by the Federal Deposit Insurance Corporation. + References No definition available. + Details
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Balance Type: | debit
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- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |