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Loans Receivable (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2014
Dec. 31, 2012
Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Age of Consumer Loans contractual payments due with no forecasted future net cash flows   120 months    
Increase in provision for credit losses $ 3.0us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease   $ 2.8us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease  
Increase in finance charges     0.8cacc_IncreaseDecreaseInFinanceCharges  
Dealer Loans [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Increase in provision for credit losses       1.2us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= cacc_DealerLoansMember
Purchased Loans [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Increase in provision for credit losses       $ 1.8us-gaap_AllowanceForLoanAndLeaseLossesPeriodIncreaseDecrease
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= cacc_PurchasedLoansMember