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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 portfolio_segment segment | Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items] |  |
Number of reportable segments | 1 us-gaap_NumberOfReportableSegments |
Uninsured cash and cash equivalents | $ 86.2 us-gaap_CashUninsuredAmount | $ 5.8 us-gaap_CashUninsuredAmount
Number of portfolio segments | 2 cacc_NumberofPortfolioSegments |
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 | $ 196.9 us-gaap_CashUninsuredAmount / us-gaap_CashAndCashEquivalentsAxis = cacc_RestrictedCashAndCashEquivalentsMember | $ 155.3 us-gaap_CashUninsuredAmount / us-gaap_CashAndCashEquivalentsAxis = cacc_RestrictedCashAndCashEquivalentsMember

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

X
- 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

X
- 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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- Details
Name: | cacc_SummaryOfSignificantAccountingPoliciesLineItems
Namespace Prefix: | cacc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- 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
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_CashAndCashEquivalentsAxis=cacc_RestrictedCashAndCashEquivalentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |