Goodwill and Identifiable Intangible Assets - Acquired Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 916,186	$ 838,159
Accumulated Amortization	(442,052)	(361,327)
Net Carrying Amount	474,134	476,832
Staffing databases
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	52,336	41,036
Accumulated Amortization	(33,398)	(24,784)
Net Carrying Amount	18,938	16,252
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	532,048	478,122
Accumulated Amortization	(210,606)	(179,795)
Net Carrying Amount	321,442	298,327
Tradenames and trademarks
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	284,488	260,140
Accumulated Amortization	(161,656)	(121,576)
Net Carrying Amount	122,832	138,564
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	9,399	7,555
Accumulated Amortization	(6,727)	(6,035)
Net Carrying Amount	2,672	1,520
Acquired technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	37,915	51,306
Accumulated Amortization	(29,665)	(29,137)
Net Carrying Amount	$ 8,250	$ 22,169