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Business Combinations - Additional Information (Detail) | 3 Months Ended | 9 Months Ended | 225 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 USD ($) Entity | Sep. 30, 2019 USD ($) Entity | Sep. 30, 2020 USD ($) Entity | Sep. 30, 2019 USD ($) Entity | Sep. 30, 2020 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |
Accretion of the discount on acquisition | $ 5,900,000 | $ 7,100,000 | $ 24,300,000 | $ 18,500,000 |
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations | $ (14,200,000) | $ 1,400,000 | $ 77,400,000 | $ 6,500,000 |
Number of companies acquired | Entity | 55 | 29 | 126 | 86 | 573
Aggregate amount of maximum earnout obligations related to acquisitions |  |  | $ 1,026,100,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  | 502,500,000 |  |
Goodwill | $ 200,700,000 |  | 200,700,000 |  | $ 200,700,000
Expiration lists | 229,200,000 |  | 229,200,000 |  | 229,200,000
Non-compete agreements | 3,600,000 |  | 3,600,000 |  | 3,600,000
Trade names | 5,000,000.0 |  | 5,000,000.0 |  | 5,000,000.0
Total revenues | 1,850,600,000 | $ 1,839,200,000 | 5,315,700,000 | $ 5,515,900,000 |
Brokerage and Risk Management [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Goodwill | 200,700,000 |  | 200,700,000 |  | 200,700,000
Expiration lists | 229,200,000 |  | 229,200,000 |  | 229,200,000
Non-compete agreements | 3,600,000 |  | 3,600,000 |  | 3,600,000
Trade names | 5,000,000.0 |  | 5,000,000.0 |  | 5,000,000.0
Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Impairment wrote off of amortizable assets | 3,700,000 |  | 49,700,000 | $ 0 |
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes | 8,800,000 |  | 8,800,000 |  | 8,800,000
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes | 100,000 |  | 100,000 |  | 100,000
Brokerage [Member] | Trade Names [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes | $ 0 |  | $ 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 |  |
2020 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
2020 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
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 6.0 |  | 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 |  | 9.0
2018 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 | 1.5 | 1.5 | 1.5
2018 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 | 4.1 | 4.1 | 4.1
2018 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 | 9.0 | 9.0 | 9.0
2018 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 | 13.5 | 13.5 | 13.5
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  |  | 5.00% | 5.00% |
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Attrition rate |  |  | 15.70% | 15.70% |
Business Acquisition [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Trade names | $ 5,000,000.0 |  | $ 5,000,000.0 |  | $ 5,000,000.0
Annualized revenue of business acquisitions |  |  |  | $ 151,200,000 |
Total revenues |  |  |  | 30,400,000 |
Net earnings |  |  |  | $ 4,300,000 |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Expiration lists | 229,200,000 |  | 229,200,000 |  | 229,200,000
Non-compete agreements | 3,600,000 |  | 3,600,000 |  | 3,600,000
Deferred tax liability | $ 2,500,000 |  | $ 2,500,000 |  | $ 2,500,000

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