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Summary of Significant Accounting Policies - Trade Receivables and Related Reserves (Details) | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022
Accounting Policies [Abstract] |  |
Accounts receivable reserves as percentage of gross accounts receivable | 1.00% | 1.00%

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- Definition Accounts receivable reserves as percentage of gross accounts receivable. + References No definition available. + Details
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Namespace Prefix: | kfrc_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | duration

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