Summary of Significant Accounting Policies - Trade Receivables and Related Reserves (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Accounts receivable reserves as percentage of gross accounts receivable	1.00%	1.00%

Accounts receivable reserves as percentage of gross accounts receivable.

No definition available.

Name:	kfrc_AccountsReceivableReservesAsPercentageOfGrossAccountsReceivable
Namespace Prefix:	kfrc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration