Goodwill and Intangible Assets - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill [Roll Forward]
Goodwill, Beginning Balance	$ 1,566,956	$ 1,652,723	$ 351,725
Goodwill, Acquired During Period	21,077	65,462	1,303,649
Goodwill, Impairment Loss		(116,714)
Disposition	(98,779)		(935)
Goodwill, Foreign Currency Translation Gain (Loss)	7,526	(25,642)	(1,716)
Other (2)	2,035	(8,873)
Goodwill. Ending Balance	1,498,815	1,566,956	1,652,723
Integrated Agencies Network | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	968,394	1,153,073	95,164
Goodwill, Acquired During Period	18,451	3,330	1,058,411
Goodwill, Impairment Loss		(49,840)
Disposition	(98,779)
Transfer of Goodwill between reporting segments	(8,517)	(111,065)
Goodwill, Foreign Currency Translation Gain (Loss)	1,846	(11,422)	(502)
Other (2)		(15,682)
Goodwill. Ending Balance	881,395	968,394	1,153,073
Brand Performance Network | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	433,758	358,613	180,639
Goodwill, Acquired During Period	2,626	26,176	178,994
Goodwill, Impairment Loss		(49,314)
Transfer of Goodwill between reporting segments		111,065
Goodwill, Foreign Currency Translation Gain (Loss)	5,056	(13,467)	(1,020)
Other (2)	220	685
Goodwill. Ending Balance	441,660	433,758	358,613
Communications Network | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	127,717	115,777	49,533
Goodwill, Acquired During Period		6,569	66,244
Transfer of Goodwill between reporting segments	8,517
Goodwill, Foreign Currency Translation Gain (Loss)	353	(753)
Other (2)		6,124
Goodwill. Ending Balance	136,587	127,717	115,777
All Other | Operating Segments
Goodwill [Roll Forward]
Goodwill, Beginning Balance	37,087	25,260	26,389
Goodwill, Acquired During Period		29,387
Goodwill, Impairment Loss		(17,560)
Disposition			(935)
Goodwill, Foreign Currency Translation Gain (Loss)	271		(194)
Other (2)	1,815
Goodwill. Ending Balance	$ 39,173	$ 37,087	$ 25,260