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Cancellation Reserves (Narrative) (Details) - USD ($) $ in Millions | Aug. 31, 2017 | Feb. 28, 2017
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Cancellation Reserves [Abstract] |  |
Cancellation reserves, current portion | $ 57.5 | $ 56.4

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- Definition Total of current allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, charged to costs and expenses. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Contract Cancellation Reserves [Abstract] + References No definition available. + Details
Name: | kmx_ContractCancellationReservesAbstract
Namespace Prefix: | kmx_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration