Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2017 USD ($)
Goodwill [Roll Forward]
Goodwill, Written off Related to Sale of Business Unit	$ 1,494
Beginning balance	2,675,402
Goodwill of acquired businesses	27,580
Ending balance	2,701,488
Services
Goodwill [Roll Forward]
Goodwill, Written off Related to Sale of Business Unit	1,494
Beginning balance	1,354,667
Goodwill of acquired businesses	18,807
Ending balance	1,371,980
National Programs
Goodwill [Roll Forward]
Goodwill, Written off Related to Sale of Business Unit	0
Beginning balance	901,294
Goodwill of acquired businesses	7,314
Ending balance	908,608
Wholesale Brokerage
Goodwill [Roll Forward]
Goodwill, Written off Related to Sale of Business Unit	0
Beginning balance	284,869
Goodwill of acquired businesses	770
Ending balance	285,639
Services
Goodwill [Roll Forward]
Goodwill, Written off Related to Sale of Business Unit	0
Beginning balance	134,572
Goodwill of acquired businesses	689
Ending balance	$ 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=64834752&loc=SL49117168-202975

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- Definition

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

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- Definition

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

+ References

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- Definition

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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