Business Combinations - Additional Information (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($) Acquisition	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	4
Aggregate purchase price of acquisitions	$ 1,800
Payments to Acquire Businesses, Gross	162,752
Net assets acquired	194,612
Goodwill, Acquired During Period	121,828
Goodwill currently deductible for income tax purposes	96,700
Goodwill related to the recorded earn-out payables	25,100
Total revenues related to acquisitions	17,500
Loss before income taxes related to acquisitions	100
Estimated acquisition earn-out payables	242,016	$ 257,566	$ 258,943	$ 218,467	$ 157,720	$ 161,513
Maximum Future Contingency payments Acquisitions	543,200
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	153,200
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	88,800
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	122,612
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	(1,337)
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ 553
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	2
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Other Acquisitions
Business Acquisition [Line Items]
Net assets acquired	$ 2,285
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 20,000