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Business Acquisitions Purchase Price Allocation Goodwill Amount

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

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Business Combinations - Additional Information (Detail) £ in Millions			3 Months Ended		207 Months Ended
	Mar. 03, 2019 GBP (£)	Dec. 22, 2018 USD ($)	Mar. 31, 2019 USD ($) Entity	Mar. 31, 2018 USD ($) Entity	Mar. 31, 2019 USD ($) Entity
Business Acquisition [Line Items]
Accretion of the discount on acquisition			$ 5,600,000	$ 5,100,000
Number of companies acquired \| Entity			33	39	518
Aggregate amount of maximum earnout obligations related to acquisitions			$ 618,900,000
Expense related to net adjustments to estimated fair value of liability for earnout obligations			2,700,000	$ 2,300,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet			293,600,000
Goodwill			131,500,000		$ 131,500,000
Expiration lists			130,600,000		130,600,000
Non-compete agreements			2,100,000		2,100,000
Total revenues related to acquisitions in the aggregate			1,993,000,000	$ 1,855,900,000
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Goodwill			131,500,000		131,500,000
Expiration lists			130,600,000		130,600,000
Non-compete agreements			$ 2,100,000		$ 2,100,000
Minimum [Member] \| Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			2 years
Minimum [Member] \| Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			3 years
Minimum [Member] \| Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years			2 years
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			5 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			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			4.4		4.4
2018 Acquisitions [Member] \| Valuation, Income Approach [Member] \| Measurement Input, Long-term Revenue Growth Rate [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Measurement input			11.5		11.5
2018 Acquisitions [Member] \| Valuation, Income Approach [Member] \| Measurement Input, Long-term Revenue Growth Rate [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Measurement input			14.0		14.0
2017 Acquisitions [Member] \| Valuation, Income Approach [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Attrition rate			3.50%
2017 Acquisitions [Member] \| Valuation, Income Approach [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Attrition rate			11.00%
Business Acquisition [Member]
Business Acquisition [Line Items]
Annualized revenue of business acquisitions			$ 71,200,000
Total revenues related to acquisitions in the aggregate			18,800,000
Net earnings			2,100,000
Business Acquisition [Member] \| Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists			130,600,000		$ 130,600,000
Non-compete agreements			2,100,000		2,100,000
Deferred tax liability			5,800,000		5,800,000
Business Acquisition [Member] \| Expiration Lists [Member] \| Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes			22,700,000		22,700,000
Business Acquisition [Member] \| Non-Compete Agreements [Member] \| Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes			$ 800,000		$ 800,000
Stackhouse Poland Group Limited [Member] \| United Kingdom, Pounds \| Definitive Agreement [Member]
Business Acquisition [Line Items]
Cash consideration		$ 350,000,000
Percentage of interest acquired in acquired entity		100.00%
Jardine Lloyd Thompson Group plc (JLT) [Member] \| Definitive Agreement [Member]
Business Acquisition [Line Items]
Cash consideration \| £	£ 130.0
Business Combination, Contingent Consideration, Liability \| £	£ 60.0
2019 Acquisitions [Member] \| Valuation, Market Approach [Member] \| Measurement Input, Discount Rate [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Measurement input			5.0		5.0
2019 Acquisitions [Member] \| Valuation, Market Approach [Member] \| Measurement Input, Discount Rate [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Measurement input			17.5		17.5
2019 Acquisitions [Member] \| Valuation, Income Approach [Member] \| Measurement Input, Discount Rate [Member]
Business Acquisition [Line Items]
Measurement input			8.0		8.0