Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 26, 2020	Dec. 28, 2019
Revenue from Contract with Customer [Abstract]
Cumulative effect of a change in accounting principle related to ASC 326	$ 203	$ 0

Amount of allowance for doubtful accounts receivable cumulative effect of change.

No definition available.

Name:	crai_AllowanceForDoubtfulAccountsReceivableCumulativeEffectOfChange
Namespace Prefix:	crai_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration