Business Combinations - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Acquisition	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	2
Aggregate purchase price of acquisitions	$ 600
Payments to Acquire Businesses, Gross	116,563
Net assets acquired	136,434
Goodwill, Acquired During Period	79,924
Goodwill currently deductible for income tax purposes	64,900
Goodwill related to the recorded earn-out payables	15,000
Total revenues related to acquisitions	8,700
Income before income taxes related to acquisitions	400
Estimated acquisition earn-out payables	257,566	$ 258,943	$ 157,720	$ 161,513
Maximum Future Contingency payments Acquisitions	558,500
Asset Purchase Agreement
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	25,000
Expected minimum calculated earn out payments	25,000
Expected maximum deemed earn out payments	$ 25,000
Expected payment period of earn out payments	will be paid in accordance with the asset purchase agreement in the first quarter of calendar year 2022
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 154,300
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	103,300
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	83,301
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	(1,337)
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ (2,040)
Purchased Customer Accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years
Non-compete Agreements
Business Acquisition [Line Items]
Weighted average life (years)	3 years
Business Combinations - Asset Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Other Acquisitions
Business Acquisition [Line Items]
Net assets acquired	$ (1,214)
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 20,000