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Reinsurance (Percentage Payout Of Incurred Claims) (Details) | 12 Months Ended
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Dec. 31, 2018
Percentage of Claims Paid During Period One [Member] |
Percentage Payment by Period [Line Items] |
Payout of Incurred Claims | 95.10%
Percentage of Claims Paid During Period Two [Member] |
Percentage Payment by Period [Line Items] |
Payout of Incurred Claims | 4.90%
Percentage of Claims Paid During Period Three [Member] |
Percentage Payment by Period [Line Items] |
Payout of Incurred Claims | 0.00%
Percentage of Claims Paid During Period Four [Member] |
Percentage Payment by Period [Line Items] |
Payout of Incurred Claims | 0.00%
Percentage of Claims Paid During Period Five [Member] |
Percentage Payment by Period [Line Items] |
Payout of Incurred Claims | 0.00%

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- Definition Payout of Incurred Claims + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition [Line Items] for Percentage Payment by Period [Table] + References No definition available. + Details
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Namespace Prefix: | cacc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cacc_PercentagePercentagePaymentbyPeriodAxis=cacc_PercentageofClaimsPaidDuringPeriodFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |