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Business Combinations - Additional Information (Detail)
shares in Millions
 3 Months Ended 9 Months Ended 237 Months Ended
Aug. 13, 2021
USD ($)
Jul. 29, 2021
May 20, 2021
USD ($)
May 17, 2021
USD ($)
shares
Sep. 30, 2021
USD ($)
Entity
Jun. 30, 2021
shares
Sep. 30, 2020
USD ($)
Entity
Sep. 30, 2021
USD ($)
Entity
Sep. 30, 2020
USD ($)
Entity
Sep. 30, 2021
USD ($)
Entity
Dec. 31, 2020
USD ($)
Business Acquisition [Line Items]                      
Net cash raised through follow-on common stock offering               $ 1,524,600,000 $ 83,600,000    
Aggregate principal amount         $ 5,532,100,000     5,532,100,000   $ 5,532,100,000 $ 4,551,600,000
Loss on extinguishment of debt $ 16,200,000       16,200,000     16,200,000      
Accretion of the discount on acquisition         8,200,000   $ 5,900,000 26,400,000 24,300,000    
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations         $ (26,000,000.0)   $ (14,200,000) $ (36,600,000) $ 77,400,000    
Number of companies acquired | Entity         44   55 80 126 602  
Aggregate amount of maximum earnout obligations related to acquisitions               $ 1,026,100,000 $ 1,026,100,000    
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet               578,700,000 578,700,000    
Aggregate amount of earnout obligation expected settlement in cash or stock at option         $ 560,500,000     560,500,000   $ 560,500,000  
Aggregate amount of earnout obligation expected settlement in cash         18,200,000     18,200,000   18,200,000  
Goodwill         453,500,000     453,500,000   453,500,000  
Expiration lists         279,900,000     279,900,000   279,900,000  
Non-compete agreements         3,800,000     3,800,000   3,800,000  
Trade names         4,700,000     4,700,000   4,700,000  
Total revenues         2,140,700,000   $ 1,899,600,000 6,278,400,000 5,435,700,000    
Brokerage and Risk Management [Member]                      
Business Acquisition [Line Items]                      
Goodwill         453,500,000     453,500,000   453,500,000  
Expiration lists         279,900,000     279,900,000   279,900,000  
Non-compete agreements         3,800,000     3,800,000   3,800,000  
Trade names         4,700,000     4,700,000   4,700,000  
Impairment wrote off of amortizable assets         0            
Brokerage [Member]                      
Business Acquisition [Line Items]                      
Impairment wrote off of amortizable assets             $ 3,700,000 13,100,000 $ 49,700,000    
Brokerage [Member] | Expiration Lists [Member]                      
Business Acquisition [Line Items]                      
Business acquisition not deductible for income tax purposes         118,000,000.0     118,000,000.0   118,000,000.0  
Brokerage [Member] | Non-Compete Agreements [Member]                      
Business Acquisition [Line Items]                      
Business acquisition not deductible for income tax purposes         1,600,000     1,600,000   1,600,000  
Brokerage [Member] | Trade Names [Member]                      
Business Acquisition [Line Items]                      
Business acquisition not deductible for income tax purposes         $ 3,500,000     $ 3,500,000   $ 3,500,000  
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      
Common Stock [Member]                      
Business Acquisition [Line Items]                      
Stock issuance from public offering (in shares) | shares           10.3          
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]                      
Business Acquisition [Line Items]                      
Aggregate principal amount     $ 1,500,000,000.0   $ 850,000,000.0     $ 850,000,000.0   $ 850,000,000.0  
Weighted average interest rate     3.31%                
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member]                      
Business Acquisition [Line Items]                      
Aggregate principal amount     $ 650,000,000.0                
Periodic payment of interest     2.50%                
Periodic payments of interest and principal, expiry year     2031                
Notes redemption date   Aug. 13, 2021                  
Loss on extinguishment of debt 16,200,000                    
Unsecured Senior Notes [Member] | Fixed Rate of 3.50%, Balloon Due May 20, 2051 [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                
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                    
Net cash raised through follow-on common stock offering 1,400,000,000                    
Cash borrowed through debt issuance and additional cash generated $ 850,000,000                    
Debt instrument term 30 years                    
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         2.5     2.5   2.5  
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, 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         4.8     4.8   4.8  
2021 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member]                      
Business Acquisition [Line Items]                      
Attrition rate               4.30%      
2021 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member]                      
Business Acquisition [Line Items]                      
Attrition rate               16.10%      
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  
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]                      
Business Acquisition [Line Items]                      
Measurement input         9.0     9.0   9.0  
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]                      
Business Acquisition [Line Items]                      
Measurement input         17.0     17.0   17.0  
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   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         9.0     9.0   9.0  
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]                      
Business Acquisition [Line Items]                      
Measurement input         17.0     17.0   17.0  
Business Acquisition [Member]                      
Business Acquisition [Line Items]                      
Trade names         $ 4,700,000     $ 4,700,000   $ 4,700,000  
Annualized revenue of business acquisitions               189,900,000      
Total revenues               95,800,000      
Net earnings               3,300,000      
Business Acquisition [Member] | Brokerage [Member]                      
Business Acquisition [Line Items]                      
Expiration lists         279,900,000     279,900,000   279,900,000  
Non-compete agreements         3,800,000     3,800,000   3,800,000  
Deferred tax liability         $ 20,200,000     $ 20,200,000   $ 20,200,000