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Debt (Summary Of Additional Information Of Credit facilities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Revolving Secured Line Of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 204.7us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember		$ 195.1us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember
Average outstanding balance	103.6us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember		97.9us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	290.2us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIiMember		169.1us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIiMember
Average outstanding balance	70.3us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIiMember		81.8us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIiMember
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	26.7us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIVMember		39.6us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIVMember
Average outstanding balance	22.0us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIVMember		11.8us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityIVMember
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	75.0us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityVMember	[1]	60.0us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityVMember	[1]
Average outstanding balance	$ 9.2us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityVMember	[1]	$ 23.3us-gaap_LineOfCreditFacilityAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = cacc_WarehouseFacilityVMember	[1]
[1] In connection with the renewal of this warehouse facility in the third quarter of 2014, we formed a new wholly owned subsidiary, CAC Warehouse Funding LLC V, which replaced CAC Warehouse Funding III, LLC.

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