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Accumulated Other Comprehensive Loss (Narrative) (Details) - USD ($) $ in Millions | Aug. 31, 2017 | Feb. 28, 2017
--- | --- | ---
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] |  |
Deferred tax | $ 35.8 | $ 33.8

X
- Definition The balance of deferred taxes on items remaining in accumulated other comprehensive income loss + References No definition available. + Details
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Namespace Prefix: | kmx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_ComprehensiveIncomeNetOfTaxAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration