Business Acquisition Purchase Price Allocation Goodwill Not Expected Tax Deductible Amount

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Business Acquisitions Purchase Price Allocation Goodwill Amount

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Business Combination ,Aggregate Earnout Obligation, Maximum

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Business Combination Aggregate Earnout Obligation Recorded Maximum

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Business Combination, Annualize Revenue

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Expiration lists

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Fair Value Inputs, Attrition Rate

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Goodwill Related to Nondeductible Amortizable Intangible Assets

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Maximum Potential Earnout Payable

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Value of input used to measure alternative investment.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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Amount after tax of pro forma income from continuing operations as if the business combination had been completed at the beginning of a period.

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

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For transactions that are recognized separately from the acquisition of assets and assumptions of liabilities in the business combination, such as related to pre-existing relationships with the acquiree, this element represents the disclosure of the amount of acquisition related costs which have been expensed.

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Amount of deferred tax liability attributable to taxable temporary differences.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Gross carrying amount before accumulated amortization as of the balance sheet date of payments made to third parties in exchange for their agreement not to engage in specified competitive practices in specified geographic territories for a specified period of time.

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Gross carrying amount before accumulated amortization as of the balance sheet date of the rights acquired through registration of a trade name to gain or protect exclusive use thereof for a reasonably expected period of economic benefit.

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The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

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The number of businesses acquired by the entity during the period.

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Business Combinations - Additional Information (Detail)		3 Months Ended		6 Months Ended		198 Months Ended
Business Combinations - Additional Information (Detail)	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