Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance	$ 2,675,402	$ 2,586,683
Goodwill of acquired businesses	42,172	93,376
Goodwill disposed of relating to sales of businesses	(1,495)	(4,657)
Ending balance	2,716,079	2,675,402
Retail
Goodwill [Roll Forward]
Beginning balance	1,354,667	1,345,636
Goodwill of acquired businesses	33,076	13,117
Goodwill disposed of relating to sales of businesses	(1,495)	(4,657)
Ending balance	1,386,248	1,354,667
National Programs
Goodwill [Roll Forward]
Beginning balance	901,294	901,866
Goodwill of acquired businesses	7,178	(1)
Goodwill disposed of relating to sales of businesses	0	0
Ending balance	908,472	901,294
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	284,869	226,961
Goodwill of acquired businesses	1,229	57,908
Goodwill disposed of relating to sales of businesses	0	0
Ending balance	286,098	284,869
Services
Goodwill [Roll Forward]
Beginning balance	134,572	112,220
Goodwill of acquired businesses	689	22,352
Goodwill disposed of relating to sales of businesses	0	0
Ending balance	$ 135,261	$ 134,572

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=64834752&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.

+ References

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