Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Beginning balance	$ 4,395,918	$ 3,746,094
Goodwill of acquired businesses	349,973	648,095	$ 328,500
Goodwill disposed of relating to sales of businesses	(3,050)	(782)
Foreign currency translation adjustments during the year	(6,013)	2,511
Ending balance	4,736,828	4,395,918	3,746,094
Retail
Goodwill [Line Items]
Beginning balance	2,650,470	2,351,291
Goodwill of acquired businesses	345,961	299,961	302,600
Goodwill disposed of relating to sales of businesses	(3,050)	(782)
Foreign currency translation adjustments during the year	(6,135)	0
Ending balance	2,987,246	2,650,470	2,351,291
National Programs
Goodwill [Line Items]
Beginning balance	1,091,122	925,541
Goodwill of acquired businesses	(1,337)	163,070	100
Goodwill disposed of relating to sales of businesses	0	0
Foreign currency translation adjustments during the year	122	2,511
Ending balance	1,089,907	1,091,122	925,541
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	483,057	298,101
Goodwill of acquired businesses	5,349	184,956	6,500
Goodwill disposed of relating to sales of businesses	0	0
Foreign currency translation adjustments during the year	0	0
Ending balance	488,406	483,057	298,101
Services
Goodwill [Line Items]
Beginning balance	171,269	171,161
Goodwill of acquired businesses	0	108	19,300
Goodwill disposed of relating to sales of businesses	0	0
Foreign currency translation adjustments during the year	0	0
Ending balance	$ 171,269	$ 171,269	$ 171,161

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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