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Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 276.4	$ 177.5
Average outstanding balance	88.4	39.1
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	201.0	201.0
Average outstanding balance	93.0	4.5
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	100.0	0.0
Average outstanding balance	4.4	0.0
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	99.0
Average outstanding balance	0.0	2.2
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	101.5	75.0
Average outstanding balance	2.3	1.7
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	0.0	125.0
Average outstanding balance	$ 0.0	$ 2.8

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

X

- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Maximum amount borrowed under the credit facility at any time during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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Namespace Prefix:
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X

- Details

Name:	us-gaap_DebtInstrumentAxis=cacc_WarehouseFacilityIIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cacc_WarehouseFacilityIVMember
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Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cacc_WarehouseFacilityVMember
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Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cacc_WarehouseFacilityVIMember
Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cacc_WarehouseFacilityVIIMember
Namespace Prefix:
Data Type:	na
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Period Type: