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Business Combinations - Additional Information (Detail) | 1 Months Ended | 3 Months Ended | 9 Months Ended | 213 Months Ended
--- | --- | --- | --- | ---
Sep. 27, 2019 USD ($) | Sep. 30, 2019 USD ($) Entity | Sep. 30, 2018 USD ($) Entity | Sep. 30, 2019 USD ($) Entity | Sep. 30, 2018 USD ($) Entity | Sep. 30, 2019 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |  |
Accretion of the discount on acquisition |  | $ 7,100,000 | $ 4,500,000 | $ 18,500,000 | $ 14,500,000 |
Number of companies acquired | Entity |  | 29 | 38 | 86 | 95 | 545
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  | $ 764,700,000 |  |
Income related to net adjustments to estimated fair value of liability for earnout obligations |  | $ 1,400,000 | $ 1,300,000 | 6,500,000 | $ 7,300,000 |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  | 419,400,000 |  |
Goodwill |  | 756,100,000 |  | 756,100,000 |  | $ 756,100,000
Expiration lists |  | 527,500,000 |  | 527,500,000 |  | 527,500,000
Non-compete agreements |  | 21,100,000 |  | 21,100,000 |  | 21,100,000
Trade names |  | 2,500,000 |  | 2,500,000 |  | 2,500,000
Total revenues related to acquisitions in the aggregate |  | 1,835,200,000 | $ 1,851,400,000 | 5,577,200,000 | $ 5,497,900,000 |
Brokerage and Risk Management [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Goodwill |  | 756,100,000 |  | 756,100,000 |  | 756,100,000
Expiration lists |  | 527,500,000 |  | 527,500,000 |  | 527,500,000
Non-compete agreements |  | 21,100,000 |  | 21,100,000 |  | 21,100,000
Trade names |  | $ 2,500,000 |  | $ 2,500,000 |  | $ 2,500,000
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 |  |  |  | 3 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 |  |  |  | 5 years |  |
Maximum [Member] | Trade Names [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  | 15 years |  |
2018 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 2.0 |  | 2.0 |  | 2.0
2018 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 3.0 |  | 3.0 |  | 3.0
2018 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 10.5 |  | 10.5 |  | 10.5
2018 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 12.5 |  | 12.5 |  | 12.5
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Minimum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Attrition rate |  |  |  | 5.00% |  |
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Attrition rate |  |  |  | 12.50% |  |
Business Acquisition [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Annualized revenue of business acquisitions |  |  |  | $ 350,800,000 |  |
Total revenues related to acquisitions in the aggregate |  |  |  | 134,200,000 |  |
Net earnings |  |  |  | 6,700,000 |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Expiration lists |  | $ 527,500,000 |  | 527,500,000 |  | $ 527,500,000
Non-compete agreements |  | 21,100,000 |  | 21,100,000 |  | 21,100,000
Deferred tax liability |  | 28,300,000 |  | 28,300,000 |  | 28,300,000
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | 124,400,000 |  | 124,400,000 |  | 124,400,000
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | 15,100,000 |  | 15,100,000 |  | 15,100,000
Business Acquisition [Member] | Trade Names [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Trade names |  | 2,500,000 |  | 2,500,000 |  | 2,500,000
Business Acquisition [Member] | Trade Names [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | $ 200,000 |  | $ 200,000 |  | $ 200,000
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 3.0 |  | 3.0 |  | 3.0
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 20.0 |  | 20.0 |  | 20.0
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 7.5 |  | 7.5 |  | 7.5
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Measurement input |  | 8.2 |  | 8.2 |  | 8.2
LSG Insurance Partners [Member] | Definitive Agreement [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Cash consideration | $ 174,000,000.0 |  |  |  |  |
Potential earnout obligation | $ 72,000,000.0 |  |  |  |  |

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