Business Combinations - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($) acquisition	Mar. 31, 2018 USD ($) acquisition	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 115,653,000
Number of acquisitions | acquisition	8	2
Aggregate purchase price of acquisitions	$ (6,800,000)
Payments to Acquire Businesses, Gross	$ 98,299,000
Document Period End Date	Mar. 31, 2019
Goodwill, Acquired During Period	$ 80,432,000
Goodwill assigned	80,432,000
Goodwill currently deductible for income tax purposes	62,200,000
Goodwill related to the recorded earn-out payables	18,200,000
Total revenues related to acquisitions	4,800,000
Income before income taxes related to acquisitions	1,000,000.0
Estimated acquisition earn-out payables	108,710,000	$ 40,600,000	$ 89,924,000	$ 36,175,000
Maximum Future Contingency payments Acquisitions	218,200,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 25,200,000
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	$ 55,117,000
Goodwill assigned	55,100,000
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	0
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	6,063,000
Goodwill assigned	6,100,000
Services [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period	19,252,000
Goodwill assigned	19,200,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 83,500,000
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	$ 15,619,000
Goodwill assigned	$ 15,911,000
Business Combinations - Stock Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	1