Business Combinations - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2016 USD ($) acquisition customer_account	Jun. 30, 2015 USD ($) acquisition	Apr. 01, 2016 USD ($)	Jan. 01, 2016 USD ($)	Apr. 01, 2015 USD ($)	Jan. 01, 2015 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | acquisition	5	7
Aggregate purchase price of acquisitions	$ 917,497
Payments to Acquire Businesses, Gross	109,384,000	$ 105,100,000
Goodwill assigned	78,664,000
Goodwill currently deductible for income tax purposes	54,920,000
Goodwill currently non-deductible for income tax purposes	21,351,000
Goodwill related to the recorded earn-out payables	2,393,000
Total revenues related to acquisitions	8,870,000
Income before income taxes related to acquisitions	1,400,000
Estimated acquisition earn-out payables	73,454,000	$ 90,113,000	$ 69,095,000	$ 78,387,000	$ 77,709,000	$ 75,283,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 38,700,000
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
Services
Business Acquisition [Line Items]
Goodwill assigned	$ 6,366,000
National Programs
Business Acquisition [Line Items]
Goodwill assigned	(1,000)
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill assigned	49,978,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned	22,321,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 34,800,000
Business Combinations - Asset Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	4
Business Combinations - Stock Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | customer_account	1