Business Combinations - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2016 USD ($) acquisition customer_account	Sep. 30, 2015 USD ($) 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	6	9
Aggregate purchase price of acquisitions	$ 917,497
Payments to Acquire Businesses, Gross	115,313,000	$ 109,900,000
Goodwill assigned	85,682,000
Goodwill currently deductible for income tax purposes	60,473,000
Goodwill currently non-deductible for income tax purposes	21,381,000
Goodwill related to the recorded earn-out payables	3,828,000
Total revenues related to acquisitions	21,452,000
Income before income taxes related to acquisitions	2,375,000
Estimated acquisition earn-out payables	61,506,000	$ 80,036,000	$ 73,447,000	$ 78,387,000	$ 90,113,000	$ 75,283,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 23,000,000
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	$ 11,854,000
National Programs
Business Acquisition [Line Items]
Goodwill assigned	(1,000)
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill assigned	51,477,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned	22,352,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 38,500,000
Business Combinations - Asset Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	5
Business Combinations - Stock Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | customer_account	1