Cancellation Reserves (Narrative) (Details) - USD ($) $ in Millions	Nov. 30, 2017	Feb. 28, 2017
Cancellation Reserves [Abstract]
Cancellation reserves, current portion	$ 57.4	$ 56.4

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.

No definition available.

Name:	kmx_CancellationReservesCurrentPortion
Namespace Prefix:	kmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant