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Business Combinations - Additional Information (Detail) |  |  | 12 Months Ended
--- | --- | --- | ---
Dec. 20, 2022 USD ($) Employee | Dec. 01, 2021 USD ($) BrokerageOperation | Dec. 31, 2022 USD ($) Entity Employee | Dec. 31, 2021 USD ($) Entity | Dec. 31, 2020 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |
Net cash raised through follow-on public offering common stock |  |  | $ 123,100,000 | $ 1,546,700,000 | $ 111,900,000
Accretion of the discount on acquisition |  |  | 61,000,000.0 | 35,700,000 | 32,500,000
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  |  | $ (22,000,000.0) | $ (83,900,000) | $ 65,400,000
Number of companies acquired | Entity |  |  | 89 | 99 | 135
Aggregate amount of maximum earnout obligations related to acquisitions |  |  | $ 1,946,200,000 | $ 1,873,900,000 |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  | 1,077,300,000 | 988,500,000 |
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  |  | 734,000,000.0 | 670,300,000 |
Aggregate amount of earnout obligation expected settlement in cash |  |  | 343,300,000 | 318,200,000 |
Goodwill |  |  | 709,900,000 |  |
Expiration lists |  |  | 564,000,000.0 |  |
Non-compete agreements |  |  | 19,500,000 |  |
Trade names |  |  | 6,100,000 |  |
Write-off of amortizable intangible assets |  |  | 2,000,000.0 | 17,600,000 | $ 51,700,000
Total revenues |  |  | 8,710,200,000 | 8,445,800,000 |
Goodwill |  |  | $ 9,489,400,000 | 8,666,200,000 | 6,127,000,000.0
Number of employees | Employee |  |  | 2,000 |  |
Revision of Prior Period, Adjustment [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Expiration lists |  |  | $ (608,000,000.0) |  |
Increase in acquired software |  |  | 59,100,000 |  |
Fair value of earnout liability |  |  | (1,600,000) |  |
Working capital adjustment |  |  | 106,000,000.0 |  |
Goodwill |  |  | 441,300,000 |  |
Brokerage Management [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Goodwill |  |  | 709,900,000 |  |
Expiration lists |  |  | 564,000,000.0 |  |
Non-compete agreements |  |  | 19,500,000 |  |
Trade names |  |  | 6,100,000 |  |
Brokerage and Risk Management [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Write-off of amortizable intangible assets |  |  | 2,000,000.0 | 17,600,000 | 51,700,000
Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Goodwill |  |  | 9,358,100,000 | $ 8,544,600,000 | $ 6,053,600,000
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  | 127,800,000 |  |
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  | 8,500,000 |  |
Brokerage [Member] | Trade Names [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  | $ 2,000,000.0 |  |
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 |  |
Willis Towers Watson PLC [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Initial gross consideration |  | $ 3,170,000,000 |  |  |
Potential additional consideration subject to certain revenue targets |  | $ 750,000,000 |  |  |
Number of remaining international deferred closings | BrokerageOperation |  | 12 |  |  |
Initial purchase consideration subject to local regulatory approval |  | $ 180,000,000 |  |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.030 |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.052 |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  |  | 5.50% |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  |  | 9.50% |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.100 |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.165 |  |
Business Acquisition [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Annualized revenue of business acquisitions |  |  | $ 246,500,000 |  |
Total revenues |  |  | 87,900,000 |  |
Net earnings |  |  | (700,000) |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Expiration lists |  |  | 564,000,000.0 |  |
Non-compete agreements |  |  | 19,500,000 |  |
Deferred tax liability |  |  | 29,600,000 |  |
2022 Acquisitions [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Trade names |  |  | $ 6,100,000 |  |
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 3.0 |  |
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 18.5 |  |
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.030 |  |
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.052 |  |
2022 Acquisitions [Member] | Valuation, Income Approach [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  |  | 5.50% |  |
2022 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  |  | 9.50% |  |
2022 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.075 |  |
2022 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.108 |  |
2022 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.100 |  |
2022 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  |  | 0.165 |  |
BCHR Holdings, L.P. [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Initial gross consideration | $ 660,000,000.0 |  |  |  |
Net of agreed seller funded expenses and net working capital | $ 585,000,000.0 |  |  |  |
Number of employees | Employee | 2,300 |  |  |  |
Number of 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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