Goodwill and Intangible Assets - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Goodwill, Beginning Balance	$ 1,652,723	$ 351,725	$ 325,185
Acquired goodwill	65,462	1,303,649	23,775
Goodwill, Impairment Loss	(116,714)
Disposition		(935)
Transfer of Goodwill between reporting segments	0
Foreign currency translation	(25,642)	(1,716)	2,765
Other (3)	(8,873)
Goodwill. Ending Balance	1,566,956	1,652,723	351,725
Integrated Agencies Network | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	1,153,073	95,164	88,094
Acquired goodwill	3,330	1,058,411	7,070
Goodwill, Impairment Loss	(49,840)
Transfer of Goodwill between reporting segments	(111,065)
Foreign currency translation	(11,422)	(502)
Other (3)	(15,682)
Goodwill. Ending Balance	968,394	1,153,073	95,164
Brand Performance Network | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	358,613	180,639	177,073
Acquired goodwill	26,176	178,994	235
Goodwill, Impairment Loss	(49,314)
Transfer of Goodwill between reporting segments	111,065
Foreign currency translation	(13,467)	(1,020)	3,331
Other (3)	685
Goodwill. Ending Balance	433,758	358,613	180,639
Communications Network | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	115,777	49,533	33,258
Acquired goodwill	6,569	66,244	16,275
Foreign currency translation	(753)
Other (3)	6,124
Goodwill. Ending Balance	127,717	115,777	49,533
All Other | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	25,260	26,389	26,760
Acquired goodwill	29,387		195
Goodwill, Impairment Loss	(17,560)
Disposition		(935)
Foreign currency translation		(194)	(566)
Goodwill. Ending Balance	$ 37,087	$ 25,260	$ 26,389