ACQUISITIONS (Narrative) (Details) $ in Thousands						3 Months Ended		12 Months Ended
	Nov. 30, 2021 USD ($)	Nov. 03, 2021 USD ($)	Jul. 23, 2021 USD ($)	Apr. 02, 2021 USD ($) specialist	Apr. 30, 2019 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2021 USD ($) business	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Contingent consideration						$ 57,249		$ 57,249	$ 7,119	$ 3,876
Revenues								3,758,144	2,659,478	2,293,798
test IO
Business Acquisition [Line Items]
Equity interest acquired					100.00%
Cash consideration					$ 17,300
Purchase price adjustment										100
Intangible assets purchase accounting adjustment										(100)
Goodwill purchase accounting adjustment							$ (200)		(226)
Intangible assets					$ 6,219
PolSource
Business Acquisition [Line Items]
Equity interest acquired				100.00%
Number of specialists in acquired entity (more than) | specialist				350
Purchase price including contingent consideration				$ 148,200
Contingent consideration				35,400
Maximum amount of earnout payable				45,000
Purchase price adjustment						10,400
Goodwill purchase accounting adjustment						11,800
Revenue of acquiree								1,400
Acquisition related costs								55,000
Intangible assets				$ 15,790
CORE
Business Acquisition [Line Items]
Equity interest acquired			100.00%
Purchase price including contingent consideration			$ 50,200
Contingent consideration			4,000
Maximum amount of earnout payable			8,100
Deferred consideration			7,800
Purchase price adjustment								2,200
Revenue of acquiree								1,200
Acquisition related costs								14,100
Intangible assets			$ 8,368
Emakina
Business Acquisition [Line Items]
Equity interest acquired	1.31%	98.69%
Cash consideration	$ 1,700	$ 143,400
Revenue of acquiree								1,000
Acquisition related costs								24,700
Intangible assets		$ 30,488
Other 2019 Acquisitions
Business Acquisition [Line Items]
Cash consideration										24,800
Maximum amount of earnout payable										$ 3,000
Goodwill purchase accounting adjustment									499
Number of completed acquisitions										4
Intangible assets										$ 7,500
Other 2019 Acquisitions | Minimum
Business Acquisition [Line Items]
Estimated future operating results, period										12 months
Other 2019 Acquisitions | Maximum
Business Acquisition [Line Items]
Estimated future operating results, period										24 months
2020 Acquisitions
Business Acquisition [Line Items]
Contingent consideration									5,300
Maximum amount of earnout payable									$ 18,600
Goodwill purchase accounting adjustment								(24)
Number of completed acquisitions									2
Intangible assets									$ 7,300
Aggregate purchase price including contingent consideration at fair value at acquisition date									22,500
Revenues									$ 6,000
Other 2021 Acquisitions
Business Acquisition [Line Items]
Contingent consideration						17,600		17,600
Maximum amount of earnout payable						30,200		30,200
Revenue of acquiree								$ 19,500
Number of completed acquisitions | business								4,000
Intangible assets						14,100		$ 14,100
Purchase price including contingent consideration						$ 65,200		$ 65,200
Other 2021 Acquisitions | Minimum
Business Acquisition [Line Items]
Estimated future operating results, period								12 months
Other 2021 Acquisitions | Maximum
Business Acquisition [Line Items]
Estimated future operating results, period								48 months