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Business Combinations - Additional Information (Detail) shares in Millions |  |  |  |  | 3 Months Ended | 9 Months Ended | 237 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 13, 2021 USD ($) | Jul. 29, 2021 | May 20, 2021 USD ($) | May 17, 2021 USD ($) shares | Sep. 30, 2021 USD ($) Entity | Jun. 30, 2021 shares | Sep. 30, 2020 USD ($) Entity | Sep. 30, 2021 USD ($) Entity | Sep. 30, 2020 USD ($) Entity | Sep. 30, 2021 USD ($) Entity | Dec. 31, 2020 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net cash raised through follow-on common stock offering |  |  |  |  |  |  |  | $ 1,524,600,000 | $ 83,600,000 |  |
Aggregate principal amount |  |  |  |  | $ 5,532,100,000 |  |  | 5,532,100,000 |  | $ 5,532,100,000 | $ 4,551,600,000
Loss on extinguishment of debt | $ 16,200,000 |  |  |  | 16,200,000 |  |  | 16,200,000 |  |  |
Accretion of the discount on acquisition |  |  |  |  | 8,200,000 |  | $ 5,900,000 | 26,400,000 | 24,300,000 |  |
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  |  |  |  | $ (26,000,000.0) |  | $ (14,200,000) | $ (36,600,000) | $ 77,400,000 |  |
Number of companies acquired | Entity |  |  |  |  | 44 |  | 55 | 80 | 126 | 602 |
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  |  |  |  |  | $ 1,026,100,000 | $ 1,026,100,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  |  |  |  |  | 578,700,000 | 578,700,000 |  |
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  |  |  |  | $ 560,500,000 |  |  | 560,500,000 |  | $ 560,500,000 |
Aggregate amount of earnout obligation expected settlement in cash |  |  |  |  | 18,200,000 |  |  | 18,200,000 |  | 18,200,000 |
Goodwill |  |  |  |  | 453,500,000 |  |  | 453,500,000 |  | 453,500,000 |
Expiration lists |  |  |  |  | 279,900,000 |  |  | 279,900,000 |  | 279,900,000 |
Non-compete agreements |  |  |  |  | 3,800,000 |  |  | 3,800,000 |  | 3,800,000 |
Trade names |  |  |  |  | 4,700,000 |  |  | 4,700,000 |  | 4,700,000 |
Total revenues |  |  |  |  | 2,140,700,000 |  | $ 1,899,600,000 | 6,278,400,000 | 5,435,700,000 |  |
Brokerage and Risk Management [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | 453,500,000 |  |  | 453,500,000 |  | 453,500,000 |
Expiration lists |  |  |  |  | 279,900,000 |  |  | 279,900,000 |  | 279,900,000 |
Non-compete agreements |  |  |  |  | 3,800,000 |  |  | 3,800,000 |  | 3,800,000 |
Trade names |  |  |  |  | 4,700,000 |  |  | 4,700,000 |  | 4,700,000 |
Impairment wrote off of amortizable assets |  |  |  |  | 0 |  |  |  |  |  |
Brokerage [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Impairment wrote off of amortizable assets |  |  |  |  |  |  | $ 3,700,000 | 13,100,000 | $ 49,700,000 |  |
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 118,000,000.0 |  |  | 118,000,000.0 |  | 118,000,000.0 |
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 1,600,000 |  |  | 1,600,000 |  | 1,600,000 |
Brokerage [Member] | Trade Names [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | $ 3,500,000 |  |  | $ 3,500,000 |  | $ 3,500,000 |
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 |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issuance from public offering (in shares) | shares |  |  |  |  |  | 10.3 |  |  |  |  |
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Periodic payment of interest |  |  |  |  | 3.50% |  |  | 3.50% |  | 3.50% |
Unsecured Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 1,500,000,000.0 |  | $ 850,000,000.0 |  |  | $ 850,000,000.0 |  | $ 850,000,000.0 |
Weighted average interest rate |  |  | 3.31% |  |  |  |  |  |  |  |
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 650,000,000.0 |  |  |  |  |  |  |  |
Periodic payment of interest |  |  | 2.50% |  |  |  |  |  |  |  |
Periodic payments of interest and principal, expiry year |  |  | 2031 |  |  |  |  |  |  |  |
Notes redemption date |  | Aug. 13, 2021 |  |  |  |  |  |  |  |  |
Loss on extinguishment of debt | 16,200,000 |  |  |  |  |  |  |  |  |  |
Unsecured Senior Notes [Member] | Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 850,000,000.0 |  | $ 850,000,000.0 |  |  | $ 850,000,000.0 |  | $ 850,000,000.0 |
Periodic payment of interest |  |  | 3.50% |  |  |  |  |  |  |  |
Periodic payments of interest and principal, expiry year |  |  | 2051 |  |  |  |  |  |  |  |
Willis Towers Watson PLC [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Initial gross consideration | 3,250,000,000 |  |  |  |  |  |  |  |  |  |
Potential additional consideration subject to certain revenue targets | 750,000,000 |  |  |  |  |  |  |  |  |  |
Net cash raised through follow-on common stock offering | 1,400,000,000 |  |  |  |  |  |  |  |  |  |
Cash borrowed through debt issuance and additional cash generated | $ 850,000,000 |  |  |  |  |  |  |  |  |  |
Debt instrument term | 30 years |  |  |  |  |  |  |  |  |  |
Agreement termination date |  | Jul. 29, 2021 |  |  |  |  |  |  |  |  |
Willis Towers Watson PLC [Member] | Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issuance from public offering (in shares) | shares |  |  |  | 10.3 |  |  |  |  |  |  |
Net proceeds after underwriting, discounts and other expense |  |  |  | $ 1,437,900,000 |  |  |  |  |  |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 2.5 |  |  | 2.5 |  | 2.5 |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 15.0 |  |  | 15.0 |  | 15.0 |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 3.0 |  |  | 3.0 |  | 3.0 |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 4.8 |  |  | 4.8 |  | 4.8 |
2021 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  |  | 4.30% |  |  |
2021 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  |  | 16.10% |  |  |
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 7.0 |  |  | 7.0 |  | 7.0 |
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 10.5 |  |  | 10.5 |  | 10.5 |
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 9.0 |  |  | 9.0 |  | 9.0 |
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 17.0 |  |  | 17.0 |  | 17.0 |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 3.0 |  |  | 3.0 |  | 3.0 |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 4.8 |  |  | 4.8 |  | 4.8 |
2020 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  |  | 4.30% |  |  |
2020 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  |  | 16.10% |  |  |
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 9.0 |  |  | 9.0 |  | 9.0 |
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 17.0 |  |  | 17.0 |  | 17.0 |
Business Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Trade names |  |  |  |  | $ 4,700,000 |  |  | $ 4,700,000 |  | $ 4,700,000 |
Annualized revenue of business acquisitions |  |  |  |  |  |  |  | 189,900,000 |  |  |
Total revenues |  |  |  |  |  |  |  | 95,800,000 |  |  |
Net earnings |  |  |  |  |  |  |  | 3,300,000 |  |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Expiration lists |  |  |  |  | 279,900,000 |  |  | 279,900,000 |  | 279,900,000 |
Non-compete agreements |  |  |  |  | 3,800,000 |  |  | 3,800,000 |  | 3,800,000 |
Deferred tax liability |  |  |  |  | $ 20,200,000 |  |  | $ 20,200,000 |  | $ 20,200,000 |

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