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Business Combinations - Additional Information (Detail) | 12 Months Ended | 216 Months Ended
--- | --- | ---
Dec. 31, 2019 USD ($) Entity | Dec. 31, 2018 USD ($) Entity | Dec. 31, 2017 USD ($) Entity | Dec. 31, 2019 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |
Accretion of the discount on acquisition | $ 27,000,000.0 | $ 18,800,000 | $ 20,200,000 |
Number of companies acquired | Entity | 116 | 112 | 108 | 556
Aggregate amount of maximum earnout obligations related to acquisitions | $ 982,900,000 | $ 558,100,000 |  |
Income related to net adjustments to estimated fair value of liability for earnout obligations | 11,700,000 | 9,200,000 | $ 10,700,000 |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet | 565,000,000.0 | 258,800,000 |  |
Goodwill | 975,700,000 |  |  | $ 975,700,000
Expiration lists | 841,300,000 |  |  | 841,300,000
Non-compete agreements | 11,700,000 |  |  | 11,700,000
Trade names | 4,000,000.0 |  |  | 4,000,000.0
Write-off of amortizable intangible assets | 100,000 | 10,600,000 | $ 6,200,000 |
Total revenues related to acquisitions in the aggregate | 7,390,500,000 | $ 7,383,100,000 |  |
Brokerage and Risk Management [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Goodwill | 975,700,000 |  |  | 975,700,000
Expiration lists | 841,300,000 |  |  | 841,300,000
Non-compete agreements | 11,700,000 |  |  | 11,700,000
Trade names | $ 4,000,000.0 |  |  | $ 4,000,000.0
Minimum [Member] | Expiration Lists [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of intangibles assets, years | 1 year |  |  |
Minimum [Member] | Non-Compete Agreements [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of intangibles assets, years | 1 year |  |  |
Minimum [Member] | Trade Names [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of intangibles assets, years | 1 year |  |  |
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 |  |  |
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 | 10.0 | 10.0 |  | 10.0
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% | 5.00% |  |
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Maximum [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Attrition rate | 12.50% | 12.50% |  |
Business Acquisition [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Annualized revenue of business acquisitions | $ 468,200,000 |  |  |
Total revenues related to acquisitions in the aggregate | 274,200,000 |  |  |
Net earnings | 24,800,000 |  |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Expiration lists | 841,300,000 |  |  | $ 841,300,000
Non-compete agreements | 11,700,000 |  |  | 11,700,000
Deferred tax liability | 43,300,000 |  |  | 43,300,000
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business acquisition not deductible for income tax purposes | 215,400,000 |  |  | 215,400,000
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business acquisition not deductible for income tax purposes | 4,700,000 |  |  | 4,700,000
Business Acquisition [Member] | Trade Names [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Trade names | 4,000,000.0 |  |  | 4,000,000.0
Business Acquisition [Member] | Trade Names [Member] | Brokerage [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business acquisition not deductible for income tax purposes | $ 400,000 |  |  | $ 400,000
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Measurement input | 4.5 |  |  | 4.5
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Measurement input | 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
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Measurement input | 9.0 |  |  | 9.0

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