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Business Combinations - Additional Information (Detail) | 3 Months Ended | 9 Months Ended | 201 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) Entity | Sep. 30, 2017 USD ($) Entity | Sep. 30, 2018 USD ($) Entity | Sep. 30, 2017 USD ($) Entity | Sep. 30, 2018 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |
Accretion of the discount on acquisition | $ 4,500,000 | $ 4,900,000 | $ 14,500,000 | $ 15,200,000 |
Expense related to net adjustments to estimated fair value of liability for earnout obligations | $ 1,300,000 | $ 5,700,000 | $ 7,300,000 | $ 12,300,000 |
Number of companies acquired | Entity | 38 | 31 | 95 | 89 | 488
Aggregate amount of maximum earnout obligations related to acquisitions | $ 495,700,000 |  | $ 495,700,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet | 219,600,000 |  | 219,600,000 |  |
Goodwill | 404,700,000 |  | 404,700,000 |  | $ 404,700,000
Expiration lists | 318,200,000 |  | 318,200,000 |  | 318,200,000
Non-compete agreements | 2,800,000 |  | 2,800,000 |  | 2,800,000
Trade names | 35,500,000 |  | 35,500,000 |  | 35,500,000
Write-off of amortizable intangible assets | 1,100,000 | $ 4,500,000 | 6,700,000 | $ 6,200,000 |
Total revenues related to acquisitions in the aggregate | 1,780,300,000 | $ 1,650,200,000 | 5,385,800,000 | $ 4,904,700,000 |
Brokerage and Risk Management [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Goodwill | 404,700,000 |  | 404,700,000 |  | 404,700,000
Expiration lists | 318,200,000 |  | 318,200,000 |  | 318,200,000
Non-compete agreements | 2,800,000 |  | 2,800,000 |  | 2,800,000
Trade names | $ 35,500,000 |  | $ 35,500,000 |  | $ 35,500,000
Expiration Lists [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  | 2 years |  |
Expiration Lists [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  | 15 years |  |
Non-Compete Agreements [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  | 3 years |  |
Non-Compete Agreements [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  | 5 years |  |
Trade Names [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  | 2 years |  |
Trade Names [Member] | Maximum [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 | 3.0 |  | 3.0 |  | 3.0
2018 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 15.0 |  | 15.0 |  | 15.0
2018 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 8.0 |  | 8.0 |  | 8.0
2018 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 9.5 |  | 9.5 |  | 9.5
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 |  |  | 10.00% | 10.00% |
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 3,000,000 | 3,000,000 | 3,000,000 | 3,000,000 | 3,000,000
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 3,500,000 | 3,500,000 | 3,500,000 | 3,500,000 | 3,500,000
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 12,000,000 | 12,000,000 | 12,000,000 | 12,000,000 | 12,000,000
2017 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Measurement input | 14,000,000 | 14,000,000 | 14,000,000 | 14,000,000 | 14,000,000
Business Acquisition [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Annualized revenue of business acquisitions |  |  | $ 249,800,000 |  |
Total revenues related to acquisitions in the aggregate |  |  | 88,300,000 |  |
Net earnings |  |  | 4,200,000 |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Expiration lists | $ 318,200,000 |  | 318,200,000 |  | $ 318,200,000
Non-compete agreements | 2,800,000 |  | 2,800,000 |  | 2,800,000
Trade names | 35,500,000 |  | 35,500,000 |  | 35,500,000
Deferred tax liability | 62,000,000 |  | 62,000,000 |  | 62,000,000
Goodwill related to nondeductible amortizable intangible assets |  |  | 62,000,000 |  |
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes | 230,700,000 |  | 230,700,000 |  | 230,700,000
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes | 2,200,000 |  | 2,200,000 |  | 2,200,000
Business Acquisition [Member] | Trade Names [Member] | Brokerage [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition not deductible for income tax purposes | $ 35,500,000 |  | $ 35,500,000 |  | $ 35,500,000

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