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Business Combinations - Additional Information (Detail) $ in Millions |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
Jan. 07, 2025 USD ($) | Dec. 19, 2024 USD ($) | Dec. 11, 2024 USD ($) | Dec. 07, 2024 USD ($) | Dec. 31, 2024 USD ($) Employee Entity | Dec. 31, 2023 USD ($) Entity | Dec. 31, 2022 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |  |  |
Cash raised through follow-on common stock offering |  |  |  |  | $ 8,506.8 | $ 120.2 | $ 123.1
Accretion of the discount on acquisition |  |  |  |  | 61.7 | 76.6 | 61.0
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  |  |  |  | $ 35.7 | $ (300.7) | $ (22.0)
Number of companies acquired | Entity |  |  |  |  | 91 | 80 | 89
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  |  | $ 1,998.2 | $ 2,009.8 |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  |  | 1,302.0 | 1,294.2 |
Aggregate amount of earnout obligation expected settlement in cash or common stock at option |  |  |  |  | 511.9 | 564.8 |
Aggregate amount of earnout obligation expected settlement in cash |  |  |  |  | 790.1 | 729.4 |
Goodwill |  |  |  |  | 837.2 |  |
Expiration lists |  |  |  |  | 791.5 |  |
Non-compete agreements |  |  |  |  | 23.6 |  |
Trade names |  |  |  |  | 1.9 |  |
Write-off of amortizable intangible assets |  |  |  |  | 19.4 | 3.5 | $ 2.0
Total revenues |  |  |  |  | 11,766.9 | 10,458.0 |
Goodwill |  |  |  |  | $ 12,270.2 | 11,475.6 | 9,489.4
Number of employees | Employee |  |  |  |  | 2,000 |  |
Brokerage Management [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  |  |  |  | $ 837.2 |  |
Expiration lists |  |  |  |  | 791.5 |  |
Non-compete agreements |  |  |  |  | 23.6 |  |
Trade names |  |  |  |  | 1.9 |  |
Brokerage [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  |  |  |  | 11,923.4 | $ 11,217.8 | $ 9,358.1
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 239.8 |  |
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 16.7 |  |
Brokerage [Member] | Trade Names [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | $ 0.5 |  |
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 |  |
2024 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.03 |  |
2024 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.19 |  |
2024 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.007 |  |
2024 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.053 |  |
2024 Acquisitions [Member] | Valuation, Income Approach [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Attrition rate |  |  |  |  | 5.00% |  |
2024 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Attrition rate |  |  |  |  | 13.50% |  |
2024 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.071 |  |
2024 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.09 |  |
2024 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.095 |  |
2024 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.115 |  |
Business Acquisition [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Annualized revenue of business acquisitions |  |  |  |  | $ 386.5 |  |
Total revenues |  |  |  |  | 169.2 |  |
Net earnings |  |  |  |  | (18.0) |  |
Net earnings before interest, income taxes, depreciation, amortization and change in estimated acquisition earnout payables |  |  |  |  | 33.1 |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Expiration lists |  |  |  |  | 791.5 |  |
Non-compete agreements |  |  |  |  | 23.6 |  |
Trade names |  |  |  |  | 1.9 |  |
Deferred tax liability |  |  |  |  | $ 64.9 |  |
2023 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.007 |  |
2023 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.053 |  |
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 |  |  |  |  | 13.50% |  |
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.095 |  |
2023 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 0.115 |  |
AssuredPartners, Inc [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Gross consideration |  |  |  | $ 13,450.0 |  |  |
Cash raised through follow-on common stock offering |  |  | $ 8,500.0 |  |  |  |
Over-Allotment Option [Member] | AssuredPartners, Inc [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Cash raised through follow-on common stock offering | $ 1,280.0 |  |  |  |  |  |
Senior Notes [Member] | AssuredPartners, Inc [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Cash borrowed from senior notes |  | $ 5,000.0 |  |  |  |  |

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