Business Combinations - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) Acquisition	Dec. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	2
Aggregate purchase price of acquisitions	$ 3.0
Payments to Acquire Businesses, Gross	479.7
Net assets acquired	490.5
Goodwill, Acquired During Period	404.6
Goodwill currently deductible for income tax purposes	4.0
Goodwill related to the recorded earn-out payables	400.6
Estimated acquisition earn-out payables	250.9	$ 291.0	$ 257.5	$ 258.9
Maximum Future Contingency payments Acquisitions	452.3
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	201.6
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	49.3
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	2.5
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	402.1
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ 0.0
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
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	$ 3.5
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 20.0