Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($)
Goodwill [Line Items]
Beginning balance	$ 3,746,094
Goodwill of acquired businesses	135,482
Goodwill disposed of relating to sales of businesses	0
Ending balance	3,881,576
Retail
Goodwill [Line Items]
Beginning balance	2,351,291
Goodwill of acquired businesses	657
Goodwill disposed of relating to sales of businesses	0
Ending balance	2,351,948
National Programs
Goodwill [Line Items]
Beginning balance	925,541
Goodwill of acquired businesses	55,607
Goodwill disposed of relating to sales of businesses	0
Ending balance	981,148
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	298,101
Goodwill of acquired businesses	79,110
Goodwill disposed of relating to sales of businesses	0
Ending balance	377,211
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://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
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

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