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Business Combinations - Additional Information (Detail)
shares in Millions
 3 Months Ended 6 Months Ended 234 Months Ended
Jul. 29, 2021
May 20, 2021
USD ($)
May 17, 2021
USD ($)
shares
May 12, 2021
USD ($)
Jun. 30, 2021
USD ($)
Entity
shares
Jun. 30, 2020
USD ($)
Entity
Jun. 30, 2021
USD ($)
Entity
Jun. 30, 2020
USD ($)
Entity
Jun. 30, 2021
USD ($)
Entity
Dec. 31, 2020
USD ($)
Business Acquisition [Line Items]                    
Cash Paid             $ 592,000,000.0      
Aggregate principal amount         $ 6,114,100,000   6,114,100,000   $ 6,114,100,000 $ 4,551,600,000
Accretion of the discount on acquisition         9,200,000 $ 7,700,000 18,200,000 $ 18,400,000    
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations         $ (4,600,000) $ (8,000,000.0) $ (10,600,000) $ 91,600,000    
Number of companies acquired | Entity         26 44 48 102 597  
Aggregate amount of maximum earnout obligations related to acquisitions             $ 1,068,100,000      
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet             560,100,000      
Aggregate amount of earnout obligation expected settlement in cash or stock at option         $ 536,200,000   536,200,000   $ 536,200,000  
Aggregate amount of earnout obligation expected settlement in cash         23,900,000   23,900,000   23,900,000  
Goodwill         387,200,000   387,200,000   387,200,000  
Expiration lists         259,500,000   259,500,000   259,500,000  
Non-compete agreements         18,200,000   18,200,000   18,200,000  
Trade names         4,100,000   4,100,000   4,100,000  
Total revenues         1,936,800,000 $ 1,620,000,000.0 4,129,400,000 $ 3,527,300,000    
Brokerage and Risk Management [Member]                    
Business Acquisition [Line Items]                    
Goodwill         387,200,000   387,200,000   387,200,000  
Expiration lists         259,500,000   259,500,000   259,500,000  
Non-compete agreements         18,200,000   18,200,000   18,200,000  
Trade names         4,100,000   4,100,000   4,100,000  
Brokerage [Member]                    
Business Acquisition [Line Items]                    
Impairment wrote off of amortizable assets         8,000,000.0 $ 200,000 13,100,000 $ 46,000,000.0    
Brokerage [Member] | Expiration Lists [Member]                    
Business Acquisition [Line Items]                    
Business acquisition not deductible for income tax purposes         138,600,000   138,600,000   138,600,000  
Brokerage [Member] | Non-Compete Agreements [Member]                    
Business Acquisition [Line Items]                    
Business acquisition not deductible for income tax purposes         16,400,000   16,400,000   16,400,000  
Brokerage [Member] | Trade Names [Member]                    
Business Acquisition [Line Items]                    
Business acquisition not deductible for income tax purposes         $ 3,300,000   $ 3,300,000   $ 3,300,000  
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      
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member]                    
Business Acquisition [Line Items]                    
Periodic payment of interest         2.50%   2.50%   2.50%  
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member]                    
Business Acquisition [Line Items]                    
Periodic payment of interest         3.50%   3.50%   3.50%  
Unsecured Senior Notes [Member] | Private Placement [Member]                    
Business Acquisition [Line Items]                    
Aggregate principal amount   $ 1,500,000,000.0     $ 1,500,000,000.0   $ 1,500,000,000.0   $ 1,500,000,000.0  
Weighted average interest rate   3.31%                
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member]                    
Business Acquisition [Line Items]                    
Aggregate principal amount   $ 650,000,000.0     650,000,000.0   650,000,000.0   650,000,000.0  
Periodic payment of interest   2.50%                
Periodic payments of interest and principal, expiry year   2031                
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member] | Subsequent Event [Member]                    
Business Acquisition [Line Items]                    
Notes redemption date Aug. 13, 2021                  
Unsecured Senior Notes [Member] | Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] | Private Placement [Member]                    
Business Acquisition [Line Items]                    
Aggregate principal amount   $ 850,000,000.0     $ 850,000,000.0   $ 850,000,000.0   $ 850,000,000.0  
Periodic payment of interest   3.50%                
Periodic payments of interest and principal, expiry year   2051                
Common Stock [Member]                    
Business Acquisition [Line Items]                    
Stock issuance from public offering (in shares) | shares         10.3          
Willis Towers Watson PLC [Member]                    
Business Acquisition [Line Items]                    
Cash Paid       $ 3,570,000,000            
Willis Towers Watson PLC [Member] | Subsequent Event [Member]                    
Business Acquisition [Line Items]                    
Agreement termination date Jul. 29, 2021                  
Willis Towers Watson PLC [Member] | Common Stock [Member]                    
Business Acquisition [Line Items]                    
Stock issuance from public offering (in shares) | shares     10.3              
Net proceeds after underwriting, discounts and other expense     $ 1,437,900,000              
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]                    
Business Acquisition [Line Items]                    
Measurement input         5.0   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   15.0  
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Minimum [Member]                    
Business Acquisition [Line Items]                    
Measurement input         7.0   7.0   7.0  
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Maximum [Member]                    
Business Acquisition [Line Items]                    
Measurement input         10.5   10.5   10.5  
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   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   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   10.0  
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]                    
Business Acquisition [Line Items]                    
Measurement input         13.5   13.5   13.5  
Business Acquisition [Member]                    
Business Acquisition [Line Items]                    
Trade names         $ 4,100,000   $ 4,100,000   $ 4,100,000  
Annualized revenue of business acquisitions             173,800,000      
Total revenues             60,600,000      
Net earnings             2,700,000      
Business Acquisition [Member] | Brokerage [Member]                    
Business Acquisition [Line Items]                    
Expiration lists         259,500,000   259,500,000   259,500,000  
Non-compete agreements         18,200,000   18,200,000   18,200,000  
Deferred tax liability         $ 26,500,000   $ 26,500,000   $ 26,500,000