Business Combinations - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) Acquisition	Dec. 31, 2021 USD ($) Acquisition	Dec. 31, 2020 USD ($) Acquisition	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	30	19
Net assets acquired	$ 2,638.0	$ 549.4	$ 893.2
Aggregate purchase price of acquisitions	(0.1)	(0.5)	3.5
Payments to Acquire Businesses, Gross	2,544.4	424.6
Goodwill, Acquired During Period	1,994.5	350.0	648.1
Goodwill currently deductible for income tax purposes	74.4	179.1	516.7
Goodwill currently non-deductible for income tax purposes	1,920.1
Goodwill related to the recorded earn-out payables	6.3	53.0	131.4
Total revenues related to acquisitions	217.1	63.8	93.9
Income before income taxes related to acquisitions	(8.8)	(10.6)	7.5
Maximum Future Contingency payments Acquisitions	542.8
Estimated acquisition earn-out payables	251.6	291.0	258.9	$ 161.5
Goodwill currently non-deductible for income tax purposes excluding related to the recorded earn-out payables	1,913.8
Other Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	132.3	212.6	179.7
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	119.3	78.4	79.2
Retail
Business Acquisition [Line Items]
Goodwill, Acquired During Period	1,366.6	346.0	300.0
National Programs
Business Acquisition [Line Items]
Goodwill, Acquired During Period	516.5	(1.3)	163.1
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill, Acquired During Period	111.4	5.3	185.0
Services
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ 0.0	$ 0.0	$ 0.1
Purchased Customer Accounts
Business Acquisition [Line Items]
Weighted average life (years)	14 years 8 months 12 days	15 years	15 years
Non-compete Agreements
Business Acquisition [Line Items]
Weighted average life (years)	4 years 1 month 6 days	5 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	12	13	20
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	11	2	1
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	7	3	4
O'Leary and Remedy
Business Acquisition [Line Items]
Goodwill currently non-deductible for income tax purposes		$ 117.9
Other Acquisitions
Business Acquisition [Line Items]
Net assets acquired		$ 29.1	$ 23.9
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Net assets acquired	$ 20.0
GRP Holdco Limited
Business Acquisition [Line Items]
Acquisition costs	$ 3.0