Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Thousands	9 Months Ended
Sep. 30, 2019 USD ($)
Goodwill [Line Items]
Beginning balance	$ 3,432,786
Goodwill of acquired businesses	275,307
Goodwill disposed of relating to sales of businesses	(5,828)
Ending balance	3,702,265
Retail
Goodwill [Line Items]
Beginning balance	2,063,150
Goodwill of acquired businesses	249,691
Goodwill disposed of relating to sales of businesses	(5,089)
Ending balance	2,307,752
National Programs
Goodwill [Line Items]
Beginning balance	926,206
Goodwill of acquired businesses	74
Goodwill disposed of relating to sales of businesses	(739)
Ending balance	925,541
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	291,622
Goodwill of acquired businesses	6,189
Goodwill disposed of relating to sales of businesses	0
Ending balance	297,811
Services
Goodwill [Line Items]
Beginning balance	151,808
Goodwill of acquired businesses	19,353
Goodwill disposed of relating to sales of businesses	0
Ending balance	$ 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.

Reference 1: 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=77989000&loc=SL49117168-202975

Reference 2: 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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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
-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=108376223&loc=d3e13816-109267

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