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Business Combinations - Additional Information (Detail)
 3 Months Ended 231 Months Ended
Mar. 31, 2021
USD ($)
Entity
Mar. 31, 2020
USD ($)
Entity
Mar. 31, 2021
USD ($)
Entity
Business Acquisition [Line Items]      
Accretion of the discount on acquisition $ 9,000,000.0 $ 10,700,000  
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations $ (6,000,000.0) $ 99,600,000  
Number of companies acquired | Entity 25 75 589
Aggregate amount of maximum earnout obligations related to acquisitions $ 1,146,100,000    
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet 623,000,000.0    
Aggregate amount of earnout obligation expected settlement in cash or stock at option 518,200,000   $ 518,200,000
Aggregate amount of earnout obligation expected settlement in cash 104,800,000   104,800,000
Goodwill 288,500,000   288,500,000
Expiration lists 196,400,000   196,400,000
Non-compete agreements 18,200,000   18,200,000
Trade names 3,200,000   3,200,000
Total revenues 2,171,500,000 $ 1,889,500,000  
Brokerage and Risk Management [Member]      
Business Acquisition [Line Items]      
Goodwill 288,500,000   288,500,000
Expiration lists 196,400,000   196,400,000
Non-compete agreements 18,200,000   18,200,000
Trade names 3,200,000   3,200,000
Brokerage [Member]      
Business Acquisition [Line Items]      
Impairment wrote off of amortizable assets 5,100,000 $ 45,800,000  
Brokerage [Member] | Expiration Lists [Member]      
Business Acquisition [Line Items]      
Business acquisition not deductible for income tax purposes 116,000,000.0   116,000,000.0
Brokerage [Member] | Non-Compete Agreements [Member]      
Business Acquisition [Line Items]      
Business acquisition not deductible for income tax purposes 16,200,000   16,200,000
Brokerage [Member] | Trade Names [Member]      
Business Acquisition [Line Items]      
Business acquisition not deductible for income tax purposes $ 3,200,000   $ 3,200,000
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 2 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 6 years    
Maximum [Member] | Trade Names [Member]      
Business Acquisition [Line Items]      
Estimated useful lives of intangibles assets, years 15 years    
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Measurement input 5.0   5.0
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Measurement input 15.0   15.0
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member]      
Business Acquisition [Line Items]      
Measurement input 9.0   9.0
2020 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
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Measurement input 4.8   4.8
2020 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member]      
Business Acquisition [Line Items]      
Attrition rate 4.30%    
2020 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member]      
Business Acquisition [Line Items]      
Attrition rate 16.10%    
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Measurement input 10.0   10.0
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Measurement input 11.5   11.5
Business Acquisition [Member]      
Business Acquisition [Line Items]      
Trade names $ 3,200,000   $ 3,200,000
Annualized revenue of business acquisitions 103,700,000    
Total revenues 19,200,000    
Net earnings 1,000,000.0    
Business Acquisition [Member] | Brokerage [Member]      
Business Acquisition [Line Items]      
Expiration lists 196,400,000   196,400,000
Non-compete agreements 18,200,000   18,200,000
Deferred tax liability $ 25,300,000   $ 25,300,000