Business Combinations - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) acquisition customer_account	Dec. 31, 2014 USD ($) acquisition	Dec. 31, 2013 USD ($) acquisition	Dec. 31, 2012 USD ($)
Business Acquisition [Line Items]
Maximum Future Contingency payments Acquisitions	$ 137,400,000
Number of acquisitions | acquisition	13	10
Aggregate purchase price of acquisitions	$ 503,442	$ 25,941	$ 504,300
Payments to Acquire Businesses, Gross	136,000,000	721,851,000	408,072,000
Goodwill assigned	136,381,000	513,925,000	295,944,000
Goodwill currently deductible for income tax purposes	91,100,000	141,900,000	41,600,000
Goodwill currently non-deductible for income tax purposes	8,400,000	338,800,000	249,200,000
Goodwill related to the recorded earn-out payables	36,900,000	33,200,000	5,100,000
Total revenues related to acquisitions	28,200,000	112,200,000	63,800,000
Income before income taxes related to acquisitions	1,500,000	(1,300,000)	900,000
Estimated acquisition earn-out payables	78,387,000	75,283,000	43,058,000	$ 52,987,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 25,300,000	$ 26,000,000	$ 6,300,000
Purchased customer accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years	15 years	15 years
Non-compete agreements
Business Acquisition [Line Items]
Weighted average life (years)	5 years	3 years 5 months	5 years
Retail
Business Acquisition [Line Items]
Goodwill assigned	$ 113,800,000	$ 86,400,000	$ 257,200,000
National Programs
Business Acquisition [Line Items]
Goodwill assigned	18,000,000	420,000,000	27,100,000
Wholesale Brokerage [Member]
Business Acquisition [Line Items]
Goodwill assigned	4,600,000	7,700,000	(800,000)
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned		(200,000)	12,400,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 53,100,000	$ 49,300,000	$ 36,800,000
Asset Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	13	9	8
Book of Business Purchases [Member]
Business Acquisition [Line Items]
Number of acquisitions	4	5	1
Stock Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	0	1	1