Business Combinations - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) acquisition customer_account	Sep. 30, 2014 USD ($) acquisition	Jul. 01, 2015 USD ($)	Jan. 01, 2015 USD ($)	Jul. 01, 2014 USD ($)	Jan. 01, 2014 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | acquisition		9	6
Number of customer accounts acquired | customer_account		4
Aggregate purchase price of acquisitions		$ 503,442
Payments to Acquire Businesses, Gross		109,926,000	$ 720,100,000
Goodwill assigned	$ 100,632,000	100,632,000
Goodwill currently deductible for income tax purposes	62,735,000	62,735,000
Goodwill currently non-deductible for income tax purposes	8,401,000	8,401,000
Goodwill related to the recorded earn-out payables	29,496,000	29,496,000
Total revenues related to acquisitions	9,218,000	16,640,000
Income before income taxes related to acquisitions	397,000	1,243,000
Estimated acquisition earn-out payables	80,036,000	80,036,000	$ 52,326,000	$ 90,113,000	$ 75,283,000	$ 52,696,000	$ 43,058,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	32,300,000	$ 32,300,000
Purchased customer accounts
Business Acquisition [Line Items]
Weighted average life (years)		15 years
Non-compete agreements
Business Acquisition [Line Items]
Weighted average life (years)		5 years
Retail
Business Acquisition [Line Items]
Goodwill assigned	78,018,000	$ 78,018,000
National Programs
Business Acquisition [Line Items]
Goodwill assigned	18,009,000	18,009,000
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill assigned	4,605,000	4,605,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 47,700,000	$ 47,700,000