Business Combinations - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($) acquisition	Sep. 30, 2017 USD ($) acquisition	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 343,150,000
Number of acquisitions | acquisition	18	6
Aggregate purchase price of acquisitions	$ 21,400
Payments to Acquire Businesses, Gross	$ 265,890,000
Document Period End Date	Sep. 30, 2018
Goodwill, Acquired During Period	$ 214,559,000
Goodwill assigned	214,559,000
Goodwill currently deductible for income tax purposes	162,900,000
Goodwill related to the recorded earn-out payables	51,700,000
Total revenues related to acquisitions	29,200,000
Income before income taxes related to acquisitions	1,800,000
Estimated acquisition earn-out payables	65,308,000	$ 34,174,000	$ 52,540,000	$ 36,175,000	$ 57,943,000	$ 63,821,000
Maximum Future Contingency payments Acquisitions	151,600,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 14,100,000
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, Acquired During Period	$ 173,025,000
Goodwill assigned	173,000,000.0
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	18,737,000
Goodwill assigned	18,700,000
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	5,524,000
Goodwill assigned	5,500,000
Services [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period	17,273,000
Goodwill assigned	17,300,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 51,200,000
Business Combinations - Asset Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	15
Other Acquisitions [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 33,523,000
Number of acquisitions | acquisition	1
Goodwill assigned	$ 20,245,000
Business Combinations - Stock Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	3