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Business Combinations - Additional Information (Detail)
 12 Months Ended
Dec. 01, 2021
USD ($)
Country
BrokerageOperation
Office
Employee
Nov. 09, 2021
USD ($)
May 20, 2021
USD ($)
May 17, 2021
USD ($)
Dec. 31, 2021
USD ($)
Entity
Dec. 31, 2020
USD ($)
Entity
Dec. 31, 2019
USD ($)
Entity
Business Acquisition [Line Items]              
Net cash raised through follow-on public offering common stock       $ 1,437,900,000 $ 1,546,700,000 $ 111,900,000 $ 101,200,000
Cash borrowed through debt issuance   $ 750,000,000          
Accretion of the discount on acquisition         35,700,000 32,500,000 27,000,000.0
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations         $ (83,900,000) $ 65,400,000 $ 11,700,000
Number of companies acquired | Entity         99 135 116
Aggregate amount of maximum earnout obligations related to acquisitions         $ 1,873,900,000 $ 1,128,100,000  
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet         988,500,000 592,200,000  
Aggregate amount of earnout obligation expected settlement in cash         318,200,000 98,500,000  
Aggregate amount of earnout obligation expected settlement in cash or stock at option         670,300,000 493,700,000  
Goodwill         2,561,500,000    
Expiration lists         2,027,800,000    
Non-compete agreements         7,300,000    
Trade names         5,200,000    
Write-off of amortizable intangible assets         17,600,000 51,700,000 $ 100,000
Total revenues         9,065,500,000 7,979,400,000  
Brokerage Management [Member]              
Business Acquisition [Line Items]              
Goodwill         2,561,500,000    
Expiration lists         2,027,800,000    
Non-compete agreements         7,300,000    
Trade names         5,200,000    
Brokerage and Risk Management [Member]              
Business Acquisition [Line Items]              
Write-off of amortizable intangible assets         17,600,000 $ 51,700,000 $ 100,000
Brokerage [Member] | Expiration Lists [Member]              
Business Acquisition [Line Items]              
Business acquisition not deductible for income tax purposes         393,000,000.0    
Brokerage [Member] | Non-Compete Agreements [Member]              
Business Acquisition [Line Items]              
Business acquisition not deductible for income tax purposes         3,900,000    
Brokerage [Member] | Trade Names [Member]              
Business Acquisition [Line Items]              
Business acquisition not deductible for income tax purposes         $ 3,800,000    
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    
30-year Senior Note [Member]              
Business Acquisition [Line Items]              
Cash borrowed through debt issuance     $ 850,000,000        
Term of notes     30 years        
10-year Note [Member]              
Business Acquisition [Line Items]              
Cash borrowed through debt issuance   $ 400,000,000          
Term of notes   10 years          
30-year Note [Member]              
Business Acquisition [Line Items]              
Cash borrowed through debt issuance   $ 350,000,000          
Term of notes   30 years          
Gallagher Re [Member]              
Business Acquisition [Line Items]              
Number of operating offices | Office 70            
Number of operating countries | Country 31            
Number of employees | Employee 2,200            
Willis Towers Watson PLC [Member]              
Business Acquisition [Line Items]              
Initial gross consideration $ 3,250,000,000            
Potential additional consideration subject to certain revenue targets $ 750,000,000            
Number of remaining international deferred closings | BrokerageOperation 12            
Initial purchase consideration subject to local regulatory approval $ 180,000,000            
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member]              
Business Acquisition [Line Items]              
Attrition rate         4.30%    
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Attrition rate         16.10%    
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]              
Business Acquisition [Line Items]              
Measurement input         2.5    
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Measurement input         15.0    
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]              
Business Acquisition [Line Items]              
Measurement input         3.0    
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Measurement input         4.8    
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]              
Business Acquisition [Line Items]              
Measurement input         7.0    
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Measurement input         10.5    
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]              
Business Acquisition [Line Items]              
Measurement input         9.0    
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Measurement input         17.0    
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member]              
Business Acquisition [Line Items]              
Attrition rate         4.30%    
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Attrition rate         16.10%    
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    
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    
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    
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]              
Business Acquisition [Line Items]              
Measurement input         17.0    
Business Acquisition [Member]              
Business Acquisition [Line Items]              
Trade names         $ 5,200,000    
Annualized revenue of business acquisitions         1,002,000,000.0    
Total revenues         188,200,000    
Net earnings         (13,000,000.0)    
Business Acquisition [Member] | Brokerage [Member]              
Business Acquisition [Line Items]              
Expiration lists         2,027,800,000    
Non-compete agreements         7,300,000    
Deferred tax liability         $ 89,800,000