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Business Combinations - Additional Information (Detail) | 3 Months Ended | 195 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) Entity | Mar. 31, 2017 USD ($) Entity | Mar. 31, 2018 USD ($) Entity
Business Acquisition [Line Items] |  |  |
Accretion of the discount on acquisition | $ 5,100,000 | $ 5,100,000 |
Expense related to net adjustments to estimated fair value of liability for earnout obligations | $ 2,300,000 | $ 6,700,000 |
Number of companies acquired | Entity | 39 | 40 | 466
Aggregate amount of maximum earnout obligations related to acquisitions | $ 571,000,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet | 270,900,000 |  |
Goodwill | 41,000,000 |  | $ 41,000,000
Expiration lists | 46,200,000 |  | 46,200,000
Non-compete agreements | 400,000 |  | 400,000
Total revenues related to acquisitions in the aggregate | 1,839,500,000 | $ 1,653,700,000 |
Brokerage and Risk Management [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Goodwill | 41,000,000 |  | 41,000,000
Expiration lists | 46,200,000 |  | 46,200,000
Non-compete agreements | $ 400,000 |  | 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 |  | 2 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] | Market Approach Valuation Technique [Member] | Minimum [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Revenue growth rate | 0.05% |  |
Discount rate | 0.08% |  |
2018 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Maximum [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Revenue growth rate | 0.15% |  |
Discount rate | 0.095% |  |
2017 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Minimum [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Revenue growth rate | 3.00% |  |
Discount rate | 12.00% |  |
Attrition rate | 5.00% |  |
2017 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Maximum [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Revenue growth rate | 3.50% |  |
Discount rate | 14.00% |  |
Attrition rate | 10.00% |  |
Business Acquisition [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Annualized revenue of business acquisitions | $ 29,900,000 |  |
Total revenues related to acquisitions in the aggregate | 6,200,000 |  |
Net earnings | 600,000 |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Expiration lists | 46,200,000 |  | 46,200,000
Non-compete agreements | 400,000 |  | 400,000
Deferred tax liability | 1,800,000 |  | 1,800,000
Goodwill related to nondeductible amortizable intangible assets | 1,800,000 |  |
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition not deductible for income tax purposes | 6,300,000 |  | 6,300,000
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition not deductible for income tax purposes | $ 100,000 |  | $ 100,000

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