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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

X
- Definition Amount of allowance for doubtful accounts receivable cumulative effect of change. + References No definition available. + Details
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Namespace Prefix: | crai_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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