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Business Combinations - Additional Information (Detail) $ in Millions |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 285 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 18, 2025 USD ($) employee | Apr. 10, 2025 USD ($) office employee | Jan. 07, 2025 USD ($) | Dec. 19, 2024 USD ($) | Dec. 11, 2024 USD ($) | Sep. 30, 2025 USD ($) acquisition | Sep. 30, 2024 USD ($) acquisition | Sep. 30, 2025 USD ($) acquisition | Sep. 30, 2024 USD ($) acquisition | Sep. 30, 2025 USD ($) company
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cash raised through follow-on common stock offering |  |  |  |  |  |  |  | $ 1,481.3 | $ 138.0 |
Accretion of the discount on acquisition |  |  |  |  |  | $ 11.9 | $ 13.8 | 36.0 | 47.6 |
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  |  |  |  |  | $ (17.0) | $ 29.1 | $ (3.1) | $ 60.2 |
Number of companies acquired |  |  |  |  |  | 41 | 29 | 82 | 74 | 776
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  |  |  | $ 1,723.1 |  | $ 1,723.1 |  | $ 1,723.1
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  |  |  | 823.5 |  | 823.5 |  | 823.5
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  |  |  |  |  | 557.9 |  | 557.9 |  | 557.9
Aggregate amount of earnout obligation expected settlement in cash |  |  |  |  |  | 265.6 |  | 265.6 |  | 265.6
Goodwill |  |  |  |  |  | 9,539.9 |  | 9,539.9 |  | 9,539.9
Expiration lists |  |  |  |  |  | 6,774.6 |  | 6,774.6 |  | 6,774.6
Non-compete agreements |  |  |  |  |  | 11.9 |  | 11.9 |  | 11.9
Trade names |  |  |  |  |  | 37.5 |  | 37.5 |  | 37.5
Total revenues |  |  |  |  |  | 3,881.5 | $ 3,646.1 | 12,505.5 | $ 11,346.0 |
Expiration Lists |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  |  | 6,641.6 |  | 6,641.6 |  | 6,641.6
Non-Compete Agreements |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  |  | 9.6 |  | 9.6 |  | 9.6
Trade Names |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  |  | 37.5 |  | 37.5 |  | 37.5
Brokerage |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Write-off of amortizable intangible assets |  |  |  |  |  | 2.4 | $ 5.4 | 43.0 | $ 19.4 |
Brokerage Management |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | 9,539.9 |  | 9,539.9 |  | 9,539.9
Expiration lists |  |  |  |  |  | 6,774.6 |  | 6,774.6 |  | 6,774.6
Non-compete agreements |  |  |  |  |  | $ 11.9 |  | $ 11.9 |  | $ 11.9
Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business combination, earnout payable evaluation period |  |  |  |  |  |  |  | 2 years |  |
Minimum | Expiration Lists |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  | 2 years |  | 2 years |  | 2 years
Minimum | Non-Compete Agreements |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  | 2 years |  | 2 years |  | 2 years
Minimum | Trade Names |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  | 2 years |  | 2 years |  | 2 years
Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business combination, earnout payable evaluation period |  |  |  |  |  |  |  | 3 years |  |
Maximum | Expiration Lists |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  | 15 years |  | 15 years |  | 15 years
Maximum | Non-Compete Agreements |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  | 6 years |  | 6 years |  | 6 years
Maximum | Trade Names |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  | 15 years |  | 15 years |  | 15 years
AssuredPartners, Inc |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Gross consideration | $ 13,800.0 |  |  |  |  |  |  |  |  |
Cash raised through follow-on common stock offering |  |  |  |  | $ 8,500.0 |  |  |  |  |
Number of employees | employee | 10,900 |  |  |  |  |  |  |  |  |
AssuredPartners, Inc | Over-Allotment Option |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cash raised through follow-on common stock offering |  |  | $ 1,300.0 |  |  |  |  |  |  |
AssuredPartners, Inc | Senior Notes |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cash borrowed from senior notes |  |  |  | $ 5,000.0 |  |  |  |  |  |
Business Acquisition |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Expiration lists |  |  |  |  |  | $ 6,774.6 |  | $ 6,774.6 |  | $ 6,774.6
Non-compete agreements |  |  |  |  |  | 11.9 |  | 11.9 |  | 11.9
Deferred tax liability |  |  |  |  |  | $ 728.3 |  | 728.3 |  | $ 728.3
Annualized revenue of business acquisitions |  |  |  |  |  |  |  | 3,427.7 |  |
Total revenues |  |  |  |  |  |  |  | 504.9 |  |
Net earnings |  |  |  |  |  |  |  | (44.4) |  |
Net earnings before interest, income taxes, depreciation, amortization and change in estimated acquisition earnout payables |  |  |  |  |  |  |  | $ 60.9 |  |
2025 Acquisitions | Valuation, Market Approach | Measurement Input, Discount Rate | Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.050 |  | 0.050 |  | 0.050
2025 Acquisitions | Valuation, Market Approach | Measurement Input, Discount Rate | Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.175 |  | 0.175 |  | 0.175
2025 Acquisitions | Valuation, Income Approach | Measurement Input, Discount Rate | Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.081 |  | 0.081 |  | 0.081
2025 Acquisitions | Valuation, Income Approach | Measurement Input, Discount Rate | Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.090 |  | 0.090 |  | 0.090
2024 Acquisitions | Valuation, Market Approach | Measurement Input, Long-term Revenue Growth Rate | Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.030 |  | 0.030 |  | 0.030
2024 Acquisitions | Valuation, Market Approach | Measurement Input, Long-term Revenue Growth Rate | Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.041 |  | 0.041 |  | 0.041
2024 Acquisitions | Valuation, Income Approach | Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  |  | 5.00% |  |
2024 Acquisitions | Valuation, Income Approach | Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  |  | 12.00% |  |
2024 Acquisitions | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate | Minimum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.095 |  | 0.095 |  | 0.095
2024 Acquisitions | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate | Maximum |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  |  | 0.120 |  | 0.120 |  | 0.120
Woodruff Sawyer & Co April 10, 2025 (WSC) |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Gross consideration |  | $ 1,200.0 |  |  |  |  |  |  |  |
Number of employees | employee |  | 600 |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | $ 811.9 |  | $ 811.9 |  | $ 811.9
Expiration lists |  |  |  |  |  | 459.2 |  | 459.2 |  | 459.2
Non-compete agreements |  |  |  |  |  | 0.0 |  | 0.0 |  | 0.0
Trade names |  |  |  |  |  | $ 8.4 |  | $ 8.4 |  | $ 8.4
Woodruff Sawyer & Co April 10, 2025 (WSC) | United States |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of offices operating | office |  | 14 |  |  |  |  |  |  |  |
Woodruff Sawyer & Co April 10, 2025 (WSC) | United Kingdom |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of offices operating | office |  | 1 |  |  |  |  |  |  |  |

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