Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Thousands	9 Months Ended
Sep. 30, 2020 USD ($)
Goodwill [Line Items]
Beginning balance	$ 3,746,094
Goodwill of acquired businesses	378,633
Goodwill disposed of relating to sales of businesses	(784)
Ending balance	4,123,943
Retail
Goodwill [Line Items]
Beginning balance	2,351,291
Goodwill of acquired businesses	114,250
Goodwill disposed of relating to sales of businesses	(784)
Ending balance	2,464,757
National Programs
Goodwill [Line Items]
Beginning balance	925,541
Goodwill of acquired businesses	178,332
Goodwill disposed of relating to sales of businesses	0
Ending balance	1,103,873
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	298,101
Goodwill of acquired businesses	85,943
Goodwill disposed of relating to sales of businesses	0
Ending balance	384,044
Services
Goodwill [Line Items]
Beginning balance	171,161
Goodwill of acquired businesses	108
Goodwill disposed of relating to sales of businesses	0
Ending balance	$ 171,269

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))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

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+ References

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
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