Business Combinations - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) acquisition	Sep. 30, 2016 USD ($) acquisition	Apr. 01, 2016 USD ($)	Jan. 01, 2016 USD ($)	Apr. 01, 2015 USD ($)	Jan. 01, 2015 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 41,178,000	$ 41,178,000
Number of acquisitions | acquisition		6	6
Aggregate purchase price of acquisitions		$ 1,406,263
Payments to Acquire Businesses, Gross		26,478,000	$ 115,300,000
Goodwill assigned	27,580,000	27,580,000
Goodwill currently deductible for income tax purposes	26,248,000	26,248,000
Goodwill related to the recorded earn-out payables	1,333,000	1,333,000
Total revenues related to acquisitions	3,191,000	3,645,000
Income before income taxes related to acquisitions	864,000	979,000
Estimated acquisition earn-out payables	34,174,000	34,174,000	$ 61,506,000	$ 57,943,000	$ 63,821,000	$ 73,447,000	$ 78,387,000
Maximum Future Contingency payments Acquisitions	81,500,000	81,500,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	28,700,000	$ 28,700,000
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	18,807,000	$ 18,807,000
National Programs
Business Acquisition [Line Items]
Goodwill assigned	7,314,000	7,314,000
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill assigned	771,000	771,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned	689,000	689,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	5,500,000	$ 5,500,000
Business Combinations - Asset Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition		6
Other Acquisitions [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	41,178,000	$ 41,178,000
Goodwill assigned	$ 27,580,000	$ 27,580,000