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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 |  |  |

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