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Business Combinations - Additional Information (Detail)
 12 Months Ended 228 Months Ended
Dec. 23, 2020
USD ($)
Employee
Office
Dec. 31, 2020
USD ($)
Entity
Dec. 31, 2019
USD ($)
Entity
Dec. 31, 2018
USD ($)
Entity
Dec. 31, 2020
USD ($)
Entity
Business Acquisition [Line Items]          
Cash consideration   $ 398,300,000      
Accrued liability   50,000,000.0     $ 50,000,000.0
Accretion of the discount on acquisition   32,500,000 $ 27,000,000.0 $ 18,800,000  
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations   $ 65,400,000 $ 11,700,000 $ 9,200,000  
Number of companies acquired | Entity   135 116 112 583
Aggregate amount of maximum earnout obligations related to acquisitions   $ 1,128,100,000 $ 982,900,000    
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet   592,200,000 565,000,000.0    
Aggregate amount of earnout obligation expected settlement in cash   98,500,000     $ 98,500,000
Aggregate amount of earnout obligation expected settlement in cash or stock at option   493,700,000     493,700,000
Goodwill   359,600,000     359,600,000
Expiration lists   467,200,000     467,200,000
Non-compete agreements   11,400,000     11,400,000
Trade names   12,500,000     12,500,000
Write-off of amortizable intangible assets   51,700,000 100,000 $ 10,600,000  
Total revenues   7,108,700,000 7,352,700,000    
Brokerage Management [Member]          
Business Acquisition [Line Items]          
Goodwill   359,600,000     359,600,000
Expiration lists   467,200,000     467,200,000
Non-compete agreements   11,400,000     11,400,000
Trade names   12,500,000     12,500,000
Brokerage and Risk Management [Member]          
Business Acquisition [Line Items]          
Write-off of amortizable intangible assets   51,700,000 $ 100,000 $ 10,600,000  
Brokerage [Member] | Expiration Lists [Member]          
Business Acquisition [Line Items]          
Business acquisition not deductible for income tax purposes   29,000,000.0     29,000,000.0
Brokerage [Member] | Non-Compete Agreements [Member]          
Business Acquisition [Line Items]          
Business acquisition not deductible for income tax purposes   600,000     600,000
Brokerage [Member] | Trade Names [Member]          
Business Acquisition [Line Items]          
Business acquisition not deductible for income tax purposes   $ 0     0
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      
The Bollington Wilson Group [Member]          
Business Acquisition [Line Items]          
Percentage of voting interest 100.00%        
Cash consideration $ 330,000,000.0        
Number of employees in the acquired company | Employee 400        
Number of operating offices in the acquired company | Office 7        
Ashton Tiffany, LLC (AT) [Member]          
Business Acquisition [Line Items]          
Cash consideration   $ 48,300,000      
Accrued liability   48,200,000     48,200,000
Goodwill   14,800,000     14,800,000
Expiration lists   78,200,000     78,200,000
Non-compete agreements   5,800,000     5,800,000
Ashton Tiffany, LLC (AT) [Member] | Common Stock [Member]          
Business Acquisition [Line Items]          
Accrued liability   $ 48,200,000     $ 48,200,000
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Measurement input   2.5     2.5
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Measurement input   15.0     15.0
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Measurement input   6.0     6.0
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Measurement input   9.0     9.0
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Attrition rate   5.00%      
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Attrition rate   15.70%      
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Measurement input   1.5     1.5
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Measurement input   4.1     4.1
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Measurement input   9.0     9.0
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Measurement input   13.5     13.5
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Attrition rate   5.00%      
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Attrition rate   15.70%      
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Measurement input   1.5     1.5
2019 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Measurement input   4.1     4.1
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]          
Business Acquisition [Line Items]          
Measurement input   9.0     9.0
2019 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]          
Business Acquisition [Line Items]          
Measurement input   13.5     13.5
Business Acquisition [Member]          
Business Acquisition [Line Items]          
Trade names   $ 12,500,000     $ 12,500,000
Annualized revenue of business acquisitions   251,400,000      
Total revenues   52,700,000      
Net earnings   5,200,000      
Business Acquisition [Member] | Brokerage [Member]          
Business Acquisition [Line Items]          
Expiration lists   467,200,000     467,200,000
Non-compete agreements   11,400,000     11,400,000
Deferred tax liability   $ 3,900,000     $ 3,900,000