Stock-Based Compensation Plans Stock-Based Compensation Plans (Schedule Of Future Share-Based Compensation Cost) (Details) $ in Millions	Sep. 30, 2025 USD ($)
Share-Based Payment Arrangement [Abstract]
Remainder of 2025	$ 12.2
2026	42.0
2027	33.5
2028	26.9
2029	18.9
Thereafter	76.8
Total Projected Stock-Based Compensation Expense	$ 210.3

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs After Four Years

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsAfterFourYears
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs In Four Years

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsInFourYears
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs In Three Years

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsInThreeYears
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs In Two Years

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsInTwoYears
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs Next Year

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsNextYear
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs Remainder of Year

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsRemainderOfYear
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Employee Service Share Based Compensation Estimated Allocation Of Recognized Period Costs Total

No definition available.

Name:	cacc_EmployeeServiceShareBasedCompensationEstimatedAllocationOfRecognizedPeriodCostsTotal
Namespace Prefix:	cacc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

No definition available.

Name:	us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration