Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015
Goodwill [Roll Forward]
Beginning balance	$ 2,460,611us-gaap_Goodwill
Goodwill of acquired businesses	29,418us-gaap_GoodwillAcquiredDuringPeriod
Goodwill disposed of relating to sales of businesses	(2,238)
Ending balance	2,487,791us-gaap_Goodwill
Retail
Goodwill [Roll Forward]
Beginning balance	1,231,869
Goodwill of acquired businesses	26,461
Goodwill disposed of relating to sales of businesses	0
Ending balance	1,258,330
National Programs
Goodwill [Roll Forward]
Beginning balance	886,095
Goodwill of acquired businesses	(194)
Goodwill disposed of relating to sales of businesses	(2,238)
Ending balance	883,663
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	222,356
Goodwill of acquired businesses	3,151
Goodwill disposed of relating to sales of businesses	0
Ending balance	225,507
Services
Goodwill [Roll Forward]
Beginning balance	120,291
Goodwill of acquired businesses	0
Goodwill disposed of relating to sales of businesses	0
Ending balance	$ 120,291

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

-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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

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

+ Details

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- Details

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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- Details

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- Details

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- Details

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- Details

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