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Debt (Schedule of Additional Information Related to Debt Instruments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	$ 188.0	$ 186.4
Average outstanding balance	62.4	56.6
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	200.1	200.1
Average outstanding balance	4.5	4.4
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	12.0	12.0
Average outstanding balance	12.0	12.0
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	100.0	40.0
Average outstanding balance	2.2	0.9
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Maximum outstanding balance	49.9	14.7
Average outstanding balance	$ 1.1	$ 8.9

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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Balance Type:
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X

- Details

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

X

- Details

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Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DebtInstrumentAxis=cacc_WarehouseFacilityVMember
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Period Type:

X

- Details

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Namespace Prefix:
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Period Type: