Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Beginning balance	$ 3,746,094	$ 3,432,786
Goodwill of acquired businesses	648,095	328,546	$ 717,700
Goodwill disposed of relating to sales of businesses	(782)	(15,238)
Foreign currency translation adjustments during the year	2,511
Ending balance	4,395,918	3,746,094	3,432,786
Retail
Goodwill [Line Items]
Beginning balance	2,351,291	2,063,150
Goodwill of acquired businesses	299,961	302,640	676,900
Goodwill disposed of relating to sales of businesses	(782)	(14,499)
Foreign currency translation adjustments during the year	0
Ending balance	2,650,470	2,351,291	2,063,150
National Programs
Goodwill [Line Items]
Beginning balance	925,541	926,206
Goodwill of acquired businesses	163,070	74	18,700
Goodwill disposed of relating to sales of businesses	0	(739)
Foreign currency translation adjustments during the year	2,511
Ending balance	1,091,122	925,541	926,206
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	298,101	291,622
Goodwill of acquired businesses	184,956	6,479	5,500
Goodwill disposed of relating to sales of businesses	0	0
Foreign currency translation adjustments during the year	0
Ending balance	483,057	298,101	291,622
Services
Goodwill [Line Items]
Beginning balance	171,161	151,808
Goodwill of acquired businesses	108	19,353
Goodwill disposed of relating to sales of businesses	0	0
Foreign currency translation adjustments during the year	0
Ending balance	$ 171,269	$ 171,161	$ 151,808

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
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-Publisher FASB
-Name Accounting Standards Codification
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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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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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