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Notes Receivable from Employees - Additional information (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) note | Dec. 31, 2019 USD ($) note | Dec. 31, 2018 USD ($)
Receivables [Abstract] |  |  |
Notes outstanding (in notes) | note | 320 | 303 |
Amortization of forgivable loan expense | $ | $ 29.4 | $ 26.3 | $ 36.4

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- Definition Amortization Of Loans To Employees + References No definition available. + Details
Name: | fcn_AmortizationOfLoansToEmployees
Namespace Prefix: | fcn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Number of Notes Receivables Outstanding + References No definition available. + Details
Name: | fcn_NumberOfNotesReceivablesOutstanding
Namespace Prefix: | fcn_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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