Business Combinations - Additional Information (Detail)	12 Months Ended			192 Months Ended
	Dec. 31, 2017 USD ($) Entity	Dec. 31, 2016 USD ($) Entity	Dec. 31, 2015 USD ($) Entity	Dec. 31, 2017 USD ($) Entity
Business Acquisition [Line Items]
Accretion of the discount on acquisition	$ 20,200,000	$ 16,900,000	$ 16,200,000
Number of companies acquired | Entity	108	101	105	459
Aggregate amount of maximum earnout obligations related to acquisitions	$ 567,900,000	$ 527,200,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet	264,200,000	242,300,000
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations	567,900,000	567,900,000	$ 567,900,000
Goodwill	304,500,000			$ 304,500,000
Expiration lists	241,400,000			241,400,000
Non-compete agreements	3,600,000			3,600,000
Trade names	100,000			100,000
Write-off of amortizable intangible assets	6,200,000	1,800,000	$ 11,500,000
Total revenues related to acquisitions in the aggregate	6,222,800,000	$ 5,765,900,000
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Goodwill	304,500,000			304,500,000
Expiration lists	241,400,000			241,400,000
Non-compete agreements	3,600,000			3,600,000
Trade names	$ 100,000			100,000
Expiration Lists [Member] | Minimum [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years	3 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	5 years
Trade Names [Member] | Maximum [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years	15 years
2017 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]
Business Acquisition [Line Items]
Revenue growth rate	0.05%
Discount rate	0.075%
2017 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]
Business Acquisition [Line Items]
Revenue growth rate	0.17%
Discount rate	0.09%
2017 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]
Business Acquisition [Line Items]
Revenue growth rate	2.00%
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	3.30%
Discount rate	14.00%
Attrition rate	10.00%
Business Acquisition [Member]
Business Acquisition [Line Items]
Annualized revenue of business acquisitions	$ 172,300,000
Total revenues related to acquisitions in the aggregate	112,600,000
Net earnings	4,700,000
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists	241,400,000			241,400,000
Non-compete agreements	3,600,000			3,600,000
Trade names	100,000			100,000
Deferred tax liability	18,600,000			18,600,000
Goodwill related to nondeductible amortizable intangible assets	18,600,000
Business Acquisition [Member] | Expiration Lists [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes	65,100,000			65,100,000
Business Acquisition [Member] | Non-Compete Agreements [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes	2,500,000			2,500,000
Business Acquisition [Member] | Trade Names [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes	$ 100,000			$ 100,000