Business Combinations - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Aggregate purchase price of acquisitions	$ 93,000
Cash payment for acquisition	720,102,000	379,694,000
Total goodwill	506,335,000
Goodwill assigned	506,306,000
Goodwill currently deductible for income tax purposes	141,757,000
Goodwill currently non-deductible for income tax purposes	351,641,000
Goodwill related to the recorded earn-out payables	12,937,000
Total revenues related to acquisitions	77,064,000
Income before income taxes related to acquisitions	5,166,000
Estimated acquisition earn-out payables	52,326,000	46,690,000	52,696,000	43,058,000	48,918,000	52,987,000
Accounts Payable [Member]
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	19,751,000
Purchased customer accounts [Member]
Business Acquisition [Line Items]
Weighted average life (years)	15 years
Non-compete agreements [Member]
Business Acquisition [Line Items]
Weighted average life (years)	3 years 3 months 18 days
Retail [Member]
Business Acquisition [Line Items]
Goodwill assigned	71,449,000
National Programs [Member]
Business Acquisition [Line Items]
Goodwill assigned	427,802,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned	7,323,000
Wholesale Brokerage [Member]
Business Acquisition [Line Items]
Goodwill assigned	(239,000)
Asset Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions	5	3
Stock Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions	1	1
Beecher Carlson Holdings, Inc. [Member]
Business Acquisition [Line Items]
Cash payment for acquisition		364,200,000
Other Non-Current Liability [Member]
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 32,575,000