Business Combinations - Additional Information (Details) $ in Millions			12 Months Ended
	Feb. 22, 2024 USD ($) Acquisition	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) Acquisition	Dec. 31, 2022 USD ($) Acquisition	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition			33	30
Aggregate purchase price of acquisitions			$ (10.9)
Cash paid		$ 4.7	695.1	$ 2,544.4
Measurement period adjustment, deferred tax liability			(17.3)
Measurement period adjustment, other current assets			5.6
Measurement period adjustment, goodwill			(12.0)
Goodwill		608.7	608.7
Goodwill currently deductible for income tax purposes		64.5	64.5
Goodwill currently non-deductible for income tax purposes		608.7	608.7
Goodwill related to the recorded earn-out payables		15.2	15.2
Total revenues related to acquisitions			67.9
Income before income taxes related to acquisitions			23.8
Maximum future contingency payments acquisitions		640.2	640.2
Estimated acquisition earn-out payables		249.2	249.2	251.6	$ 291.0	$ 258.9
Goodwill currently non-deductible for income tax purposes excluding related to the recorded earn-out payables		529.0	529.0
Goodwill after deducted for income tax		544.2	544.2
Subsequent Event
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	2
Cash paid	$ 37.6
Other Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables		103.3	103.3	132.3
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables		145.9	145.9	$ 119.3
Retail
Business Acquisition [Line Items]
Measurement period adjustment, goodwill			13.1
Goodwill net			348.5
National Programs
Business Acquisition [Line Items]
Measurement period adjustment, goodwill			(23.5)
Goodwill net			239.0
Wholesale Brokerage
Business Acquisition [Line Items]
Measurement period adjustment, goodwill			(1.6)
Goodwill net			21.2
Services
Business Acquisition [Line Items]
Measurement period adjustment, goodwill			$ 0.0
Assets and Assumed Certain Liabilities
Business Acquisition [Line Items]
Number of acquisitions | Acquisition			7
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition			22
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition			4
GRP Holdco Limited
Business Acquisition [Line Items]
Maximum potential earn- out payable		0.0	$ 0.0
Kentro Capital Limited
Business Acquisition [Line Items]
Acquisition date			Oct. 01, 2023
Kentro Capital Limited | Retail
Business Acquisition [Line Items]
Goodwill		$ 358.7	$ 358.7