Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) $ in Thousands	6 Months Ended
Jun. 30, 2016 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 2,586,683
Goodwill of acquired businesses	78,664
Goodwill, Transfers	0
Ending balance	2,665,347
Services
Goodwill [Roll Forward]
Beginning balance	1,345,636
Goodwill of acquired businesses	6,366
Goodwill, Transfers	571
Ending balance	1,352,573
National Programs
Goodwill [Roll Forward]
Beginning balance	901,866
Goodwill of acquired businesses	(1)
Goodwill, Transfers	(571)
Ending balance	901,294
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	226,961
Goodwill of acquired businesses	49,978
Goodwill, Transfers	0
Ending balance	276,939
Services
Goodwill [Roll Forward]
Beginning balance	112,220
Goodwill of acquired businesses	22,321
Goodwill, Transfers	0
Ending balance	$ 134,541

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 transfers into (out of) an asset representing the 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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