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Business Combinations - Additional Information (Detail)
$ in Millions
 3 Months Ended 9 Months Ended 249 Months Ended
Sep. 29, 2022
USD ($)
Dec. 01, 2021
USD ($)
Entity
Office
BrokerageOperation
Employee
Nov. 09, 2021
USD ($)
May 20, 2021
USD ($)
May 17, 2021
USD ($)
Sep. 30, 2022
USD ($)
Entity
Sep. 30, 2021
USD ($)
Entity
Sep. 30, 2022
USD ($)
Entity
Sep. 30, 2021
USD ($)
Entity
Sep. 30, 2022
USD ($)
Entity
Dec. 31, 2021
USD ($)
Business Acquisition [Line Items]                      
Net cash raised through follow-on common stock offering         $ 1,437.9     $ 102.2 $ 1,524.6    
Cash borrowed through debt issuance     $ 750.0                
Accretion of the discount on acquisition           $ 19.4 $ 8.2 42.8 26.4    
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations           $ 35.6 $ (26.0) $ 71.9 $ (36.6)    
Number of companies acquired | Entity           27 44 70 80 641  
Aggregate amount of maximum earnout obligations related to acquisitions               $ 1,712.9      
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet               816.3      
Aggregate amount of earnout obligation expected settlement in cash or stock at option           $ 541.8   541.8   $ 541.8  
Aggregate amount of earnout obligation expected settlement in cash           274.5   274.5   274.5  
Goodwill           283.1   283.1   283.1  
Expiration lists           213.3   213.3   213.3  
Goodwill           8,837.2   8,837.2   8,837.2 $ 8,666.2
Non-compete agreements           8.4   8.4   8.4  
Trade names           2.3   2.3   2.3  
Total revenues           2,046.0 $ 2,165.1 6,560.0 $ 6,313.3    
Brokerage Management [Member]                      
Business Acquisition [Line Items]                      
Goodwill           283.1   283.1   283.1  
Expiration lists           213.3   213.3   213.3  
Non-compete agreements           8.4   8.4   8.4  
Trade names           2.3   2.3   2.3  
Brokerage and Risk Management [Member]                      
Business Acquisition [Line Items]                      
Write-off of amortizable intangible assets               0.4 $ 13.1    
Brokerage [Member]                      
Business Acquisition [Line Items]                      
Goodwill           8,707.1   8,707.1   8,707.1 $ 8,544.6
Brokerage [Member] | Expiration Lists [Member]                      
Business Acquisition [Line Items]                      
Business acquisition not deductible for income tax purposes           103.0   103.0   103.0  
Brokerage [Member] | Non-Compete Agreements [Member]                      
Business Acquisition [Line Items]                      
Business acquisition not deductible for income tax purposes           7.2   7.2   7.2  
Brokerage [Member] | Trade Names [Member]                      
Business Acquisition [Line Items]                      
Business acquisition not deductible for income tax purposes           2.0   $ 2.0   2.0  
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      
Revision of Prior Period, Adjustment [Member]                      
Business Acquisition [Line Items]                      
Expiration lists           (608.0)   $ (608.0)   (608.0)  
Increase in acquired software           59.1   59.1   59.1  
Fair value of earnout liability           (1.6)   (1.6)   (1.6)  
Working capital adjustment           106.0   106.0   106.0  
Goodwill           $ 441.3   441.3   $ 441.3  
30-year Senior Note [Member]                      
Business Acquisition [Line Items]                      
Cash borrowed through debt issuance       $ 850.0              
Term of notes       30 years              
10-year Note [Member]                      
Business Acquisition [Line Items]                      
Cash borrowed through debt issuance     $ 400.0                
Term of notes     10 years                
30-year Note [Member]                      
Business Acquisition [Line Items]                      
Cash borrowed through debt issuance     $ 350.0                
Term of notes     30 years                
Willis Towers Watson PLC [Member]                      
Business Acquisition [Line Items]                      
Initial gross consideration   $ 3,250.0                  
Potential additional consideration subject to certain revenue targets   $ 750.0                  
Number of remaining international deferred closings | BrokerageOperation   1                  
Initial number of international deferred closings | BrokerageOperation   12                  
M&T Insurance Agency Inc [Member]                      
Business Acquisition [Line Items]                      
Percentage of equity acquired 100.00%                    
Cash consideration $ 170.0                    
Potential Earnout Obligation $ 22.5                    
2022 Acquisitions [Member]                      
Business Acquisition [Line Items]                      
Cash consideration               $ 391.3      
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]                      
Business Acquisition [Line Items]                      
Measurement input           3.0   3.0   3.0  
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]                      
Business Acquisition [Line Items]                      
Measurement input           14.0   14.0   14.0  
2022 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member]                      
Business Acquisition [Line Items]                      
Measurement input           9.0   9.0   9.0  
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   3.0   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           5.2   5.2   5.2  
2021 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member]                      
Business Acquisition [Line Items]                      
Attrition rate               5.50%      
2021 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member]                      
Business Acquisition [Line Items]                      
Attrition rate               9.50%      
2021 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   10.0  
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]                      
Business Acquisition [Line Items]                      
Measurement input           16.5   16.5   16.5  
Business Acquisition [Member]                      
Business Acquisition [Line Items]                      
Annualized revenue of business acquisitions               $ 105.2      
Total revenues               40.9      
Net earnings               (2.1)      
Business Acquisition [Member] | Brokerage [Member]                      
Business Acquisition [Line Items]                      
Expiration lists           $ 213.3   213.3   $ 213.3  
Non-compete agreements           8.4   8.4   8.4  
Deferred tax liability           $ 22.5   $ 22.5   $ 22.5  
Gallagher Re [Member]                      
Business Acquisition [Line Items]                      
Number of operating offices | Office   70                  
Number of operating countries | Entity   31                  
Number of employees | Employee   2,200