Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning balance	$ 2,716,079	$ 2,675,402
Goodwill of acquired businesses	717,710	42,172
Goodwill disposed of relating to sales of businesses	(1,003)	(1,495)
Ending balance	3,432,786	2,716,079
Retail
Goodwill [Roll Forward]
Beginning balance	1,386,248	1,354,667
Goodwill of acquired businesses	676,902	33,076
Goodwill disposed of relating to sales of businesses	0	(1,495)
Ending balance	2,063,150	1,386,248
National Programs
Goodwill [Roll Forward]
Beginning balance	908,472	901,294
Goodwill of acquired businesses	18,737	7,178
Goodwill disposed of relating to sales of businesses	(1,003)	0
Ending balance	926,206	908,472
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	286,098	284,869
Goodwill of acquired businesses	5,524	1,229
Goodwill disposed of relating to sales of businesses	0	0
Ending balance	291,622	286,098
Services
Goodwill [Roll Forward]
Beginning balance	135,261	134,572
Goodwill of acquired businesses	16,547	689
Goodwill disposed of relating to sales of businesses	0	0
Ending balance	$ 151,808	$ 135,261

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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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