Business Combinations - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) acquisition	Jun. 30, 2017 USD ($) acquisition	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 188,408,000
Number of acquisitions | acquisition	8	3
Aggregate purchase price of acquisitions	$ 21,400
Payments to Acquire Businesses, Gross	$ 149,992,000
Document Period End Date	Jun. 30, 2018
Goodwill, Acquired During Period	$ 129,775,000
Goodwill assigned	129,775,000
Goodwill currently deductible for income tax purposes	107,700,000
Goodwill related to the recorded earn-out payables	22,100,000
Total revenues related to acquisitions	7,900,000
Income before income taxes related to acquisitions	1,000,000.0
Estimated acquisition earn-out payables	52,540,000	$ 57,943,000	$ 40,600,000	$ 36,175,000	$ 57,408,000	$ 63,821,000
Maximum Future Contingency payments Acquisitions	106,100,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 24,000,000.0
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	$ 124,230,000
Goodwill assigned	124,200,000
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	0
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	5,545,000
Goodwill assigned	5,600,000
Services [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period	0
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 28,500,000
Business Combinations - Asset Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	7
Other Acquisitions [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 14,057,000
Goodwill assigned	$ 9,017,000
Business Combinations - Stock Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	1