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Business Combinations - Additional Information (Detail) |  | 3 Months Ended | 6 Months Ended | 198 Months Ended
--- | --- | --- | --- | ---
Jun. 26, 2018 USD ($) | Jun. 30, 2018 USD ($) Entity | Jun. 30, 2017 USD ($) Entity | Jun. 30, 2018 USD ($) Entity | Jun. 30, 2017 USD ($) Entity | Jun. 30, 2018 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |  |
Maximum Potential Earnout Payable |  | $ 71,100,000 |  | $ 71,100,000 |  | $ 71,100,000
Percentage of interest acquired in acquired entity | 100.00% |  |  |  |  |
Business acquisition, consideration paid in shares | $ 24,400,000 |  |  | 15,900,000 |  |
Accretion of the discount on acquisition |  | 4,900,000 | $ 5,200,000 | 10,000,000 | $ 10,300,000 |
Expense related to net adjustments to estimated fair value of liability for earnout obligations |  | $ 10,900,000 | $ 100,000 | $ 8,600,000 | $ 6,600,000 |
Number of companies acquired | Entity |  | 49 | 46 | 76 | 65 | 478
Aggregate amount of maximum earnout obligations related to acquisitions |  | $ 552,300,000 |  | $ 552,300,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  | 266,900,000 |  | 266,900,000 |  |
Goodwill |  | 294,500,000 |  | 294,500,000 |  | $ 294,500,000
Expiration lists |  | 204,000,000 |  | 204,000,000 |  | 204,000,000
Non-compete agreements |  | 1,700,000 |  | 1,700,000 |  | 1,700,000
Trade names |  | 35,400,000 |  | 35,400,000 |  | 35,400,000
Write-off of amortizable intangible assets |  | 5,600,000 | $ 1,700,000 | 5,600,000 | $ 1,700,000 |
Total revenues related to acquisitions in the aggregate |  | 1,683,400,000 | $ 1,525,800,000 | 3,560,700,000 | $ 3,211,600,000 |
Brokerage and Risk Management [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Goodwill |  | 294,500,000 |  | 294,500,000 |  | 294,500,000
Expiration lists |  | 204,000,000 |  | 204,000,000 |  | 204,000,000
Non-compete agreements |  | 1,700,000 |  | 1,700,000 |  | 1,700,000
Trade names |  | $ 35,400,000 |  | $ 35,400,000 |  | $ 35,400,000
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 |  |  |  | $ 175,100,000 |  |
Total revenues related to acquisitions in the aggregate |  |  |  | 26,700,000 |  |
Net earnings |  |  |  | 3,200,000 |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Expiration lists |  | $ 204,000,000 |  | 204,000,000 |  | $ 204,000,000
Non-compete agreements |  | 1,700,000 |  | 1,700,000 |  | 1,700,000
Trade names |  | 35,400,000 |  | 35,400,000 |  | 35,400,000
Deferred tax liability |  | 35,800,000 |  | 35,800,000 |  | 35,800,000
Goodwill related to nondeductible amortizable intangible assets |  |  |  | 35,800,000 |  |
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | 132,700,000 |  | 132,700,000 |  | 132,700,000
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | 1,100,000 |  | 1,100,000 |  | 1,100,000
Business Acquisition [Member] | Trade Names [Member] | Brokerage [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  | $ 35,400,000 |  | $ 35,400,000 |  | $ 35,400,000
DiBrina Group (DBG) [Member] | Canada [Member] | Definitive Agreement [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Cash consideration | 97,700,000 |  |  |  |  |
Maximum Potential Earnout Payable | $ 21,500,000 |  |  |  |  |

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