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Business Combinations - Additional Information (Detail) |  | 12 Months Ended | 228 Months Ended
--- | --- | --- | ---
Dec. 23, 2020 USD ($) Employee Office | Dec. 31, 2020 USD ($) Entity | Dec. 31, 2019 USD ($) Entity | Dec. 31, 2018 USD ($) Entity | Dec. 31, 2020 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |
Cash consideration |  | $ 398,300,000 |  |  |
Accrued liability |  | 50,000,000.0 |  |  | $ 50,000,000.0
Accretion of the discount on acquisition |  | 32,500,000 | $ 27,000,000.0 | $ 18,800,000 |
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  | $ 65,400,000 | $ 11,700,000 | $ 9,200,000 |
Number of companies acquired | Entity |  | 135 | 116 | 112 | 583
Aggregate amount of maximum earnout obligations related to acquisitions |  | $ 1,128,100,000 | $ 982,900,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  | 592,200,000 | 565,000,000.0 |  |
Aggregate amount of earnout obligation expected settlement in cash |  | 98,500,000 |  |  | $ 98,500,000
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  | 493,700,000 |  |  | 493,700,000
Goodwill |  | 359,600,000 |  |  | 359,600,000
Expiration lists |  | 467,200,000 |  |  | 467,200,000
Non-compete agreements |  | 11,400,000 |  |  | 11,400,000
Trade names |  | 12,500,000 |  |  | 12,500,000
Write-off of amortizable intangible assets |  | 51,700,000 | 100,000 | $ 10,600,000 |
Total revenues |  | 7,108,700,000 | 7,352,700,000 |  |
Brokerage Management [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Goodwill |  | 359,600,000 |  |  | 359,600,000
Expiration lists |  | 467,200,000 |  |  | 467,200,000
Non-compete agreements |  | 11,400,000 |  |  | 11,400,000
Trade names |  | 12,500,000 |  |  | 12,500,000
Brokerage and Risk Management [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Write-off of amortizable intangible assets |  | 51,700,000 | $ 100,000 | $ 10,600,000 |
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | 29,000,000.0 |  |  | 29,000,000.0
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | 600,000 |  |  | 600,000
Brokerage [Member] | Trade Names [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | $ 0 |  |  | 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 |  |  |
The Bollington Wilson Group [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Percentage of voting interest | 100.00% |  |  |  |
Cash consideration | $ 330,000,000.0 |  |  |  |
Number of employees in the acquired company | Employee | 400 |  |  |  |
Number of operating offices in the acquired company | Office | 7 |  |  |  |
Ashton Tiffany, LLC (AT) [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Cash consideration |  | $ 48,300,000 |  |  |
Accrued liability |  | 48,200,000 |  |  | 48,200,000
Goodwill |  | 14,800,000 |  |  | 14,800,000
Expiration lists |  | 78,200,000 |  |  | 78,200,000
Non-compete agreements |  | 5,800,000 |  |  | 5,800,000
Ashton Tiffany, LLC (AT) [Member] | Common Stock [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Accrued liability |  | $ 48,200,000 |  |  | $ 48,200,000
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 2.5 |  |  | 2.5
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 15.0 |  |  | 15.0
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 6.0 |  |  | 6.0
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 9.0 |  |  | 9.0
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  | 5.00% |  |  |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  | 15.70% |  |  |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 1.5 |  |  | 1.5
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 4.1 |  |  | 4.1
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 9.0 |  |  | 9.0
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 13.5 |  |  | 13.5
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  | 5.00% |  |  |
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  | 15.70% |  |  |
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 1.5 |  |  | 1.5
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 4.1 |  |  | 4.1
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 9.0 |  |  | 9.0
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input |  | 13.5 |  |  | 13.5
Business Acquisition [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Trade names |  | $ 12,500,000 |  |  | $ 12,500,000
Annualized revenue of business acquisitions |  | 251,400,000 |  |  |
Total revenues |  | 52,700,000 |  |  |
Net earnings |  | 5,200,000 |  |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Expiration lists |  | 467,200,000 |  |  | 467,200,000
Non-compete agreements |  | 11,400,000 |  |  | 11,400,000
Deferred tax liability |  | $ 3,900,000 |  |  | $ 3,900,000

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