Accumulated Other Comprehensive Loss (Narrative) (Details) - USD ($) $ in Millions	Nov. 30, 2017	Feb. 28, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Deferred tax	$ 30.4	$ 33.8

The balance of deferred taxes on items remaining in accumulated other comprehensive income loss

No definition available.

Name:	kmx_AccumulatedOtherComprehensiveIncomeLossDeferredTaxes
Namespace Prefix:	kmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant