Business Combinations - Additional Information (Detail) $ in Millions			12 Months Ended
	Nov. 30, 2023 State Office	Apr. 03, 2023 Employee	Dec. 31, 2023 USD ($) Entity Employee	Dec. 31, 2022 USD ($) Entity	Dec. 31, 2021 USD ($) Entity
Business Acquisition [Line Items]
Net cash raised through follow-on public offering common stock			$ 120.2	$ 123.1	$ 1,546.7
Accretion of the discount on acquisition			76.6	61.0	35.7
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations			$ (300.7)	$ (22.0)	$ (83.9)
Number of companies acquired | Entity			80	89	99
Aggregate amount of maximum earnout obligations related to acquisitions			$ 2,009.8	$ 1,946.2
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet			1,294.2	1,077.3
Aggregate amount of earnout obligation expected settlement in cash or stock at option			564.8	734.0
Aggregate amount of earnout obligation expected settlement in cash			729.4	343.3
Goodwill			1,789.2
Expiration lists			1,721.0
Non-compete agreements			30.5
Trade names			64.3
Write-off of amortizable intangible assets			3.5	2.0	$ 17.6
Total revenues			10,579.8	9,390.7
Goodwill			$ 11,475.6	9,489.4	8,666.2
Number of employees | Employee			2,000
Revision of Prior Period, Adjustment [Member]
Business Acquisition [Line Items]
Expiration lists				(608.0)
Increase in acquired software				59.1
Fair value of earnout liability				(1.6)
Working capital adjustment				106.0
Goodwill				441.3
Brokerage Management [Member]
Business Acquisition [Line Items]
Goodwill			$ 1,789.2
Expiration lists			1,721.0
Non-compete agreements			30.5
Trade names			64.3
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Write-off of amortizable intangible assets			3.5	2.0	17.6
Brokerage [Member]
Business Acquisition [Line Items]
Goodwill			11,217.8	$ 9,358.1	$ 8,544.6
Brokerage [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes			580.0
Brokerage [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes			22.9
Brokerage [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes			$ 62.8
Minimum [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			2 years
Minimum [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			2 years
Minimum [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			2 years
Maximum [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			15 years
Maximum [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			6 years
Maximum [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			15 years
Cadence Insurance, Inc. [Member]
Business Acquisition [Line Items]
Number of offices operating | Office	34
Number of states operating | State	9
Goodwill			$ 314.7
Expiration lists			499.5
Non-compete agreements			1.6
Business Acquisition [Member]
Business Acquisition [Line Items]
Annualized revenue of business acquisitions			885.1
Total revenues			394.0
Net earnings			(21.4)
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists			1,721.0
Non-compete agreements			30.5
Trade names			64.3
Deferred tax liability			$ 172.0
2023 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input			0.05
2023 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input			0.20
2023 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input			0.03
2023 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input			0.05
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Minimum [Member]
Business Acquisition [Line Items]
Attrition rate			5.00%
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member]
Business Acquisition [Line Items]
Attrition rate			25.50%
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input			0.067
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input			0.096
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input			0.09
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input			0.15
BCHR Holdings, L.P. [Member]
Business Acquisition [Line Items]
Number of employees | Employee		2,300
Number of employees with credentialed actuaries | Employee		220
BCHR Holdings, L.P. [Member] | Maximum [Member]
Business Acquisition [Line Items]
Business combination period of existence		100 years