Business Combinations - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) Acquisition	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	11
Aggregate purchase price of acquisitions	$ 1,500
Payments to Acquire Businesses, Gross	224,141
Net assets acquired	281,805
Goodwill, Acquired During Period	176,658
Goodwill currently deductible for income tax purposes	151,500
Goodwill related to the recorded earn-out payables	25,200
Total revenues related to acquisitions	28,900
Income before income taxes related to acquisitions	500
Estimated acquisition earn-out payables	262,914	$ 242,016	$ 258,943	$ 252,296	$ 218,467	$ 161,513
Maximum Future Contingency payments Acquisitions	512,900
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	168,700
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	94,200
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	174,536
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	(1,337)
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ 3,459
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
Business Combinations - Asset Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Assets and Assumed Certain Liabilities
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	8
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	2
Other Acquisitions
Business Acquisition [Line Items]
Net assets acquired	$ 11,922
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 20,000