Acquisitions Schedule of Purchase Price Allocation (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,191,270	$ 1,191,259	$ 1,085,806
H2M Group [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	29
Receivables	4,187
Prepaid expenses	188
Other current assets	5
Goodwill	25,089
Other intangible assets	11,900
Operating lease right of use assets	152
Property and equipment	56
Other assets	7
Accounts payable and accrued expenses	(1,956)
Accrued salaries and related expenses	(1,023)
Operating lease obligations—long term	(152)
Net assets acquired and liabilities assumed	38,482
Kforce Government Solutions [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	154
Receivables	17,071
Prepaid expenses	368
Other current assets	168
Goodwill	80,374
Other intangible assets	34,839
Property and equipment	361
Accounts payable and accrued expenses	(6,895)
Accrued salaries and related expenses	(4,421)
Deferred income taxes	(7,087)
Other long-term liabilities	(379)
Net assets acquired and liabilities assumed	$ 114,553