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Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]        
Net operating losses in U.K. additional cash tax payments     $ 49.0 $ 28.4
Cash and cash equivalents - non-restricted cash $ 14,758.7 $ 780.9 $ 539.4  
Stock issuance 8,344.4      
Money Market Accounts [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Total cash and cash equivalents, restricted cash and fiduciary cash 15,371.6 1,744.9    
Stock issuance 8,500.0      
Senior notes 5,000.0      
Money Market Accounts [Member] | Investment Income [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Increases dividend income 105.9      
Proceeds from AssuredPartners financing 29.0      
Dividend income $ 473.2 $ 367.3    
Qualified Contributory Savings and Thrift 401(k) Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Matching contributions by employer, percentage 100.00%      
Percentage of eligible compensation for matching contributions by employer 5.00% 5.00% 5.00%  
Matching contributions vesting schedule 5 years      
Contribution expense to plan $ 105.4 $ 86.0 $ 73.8  
Employer match on eligible compensation 5.00% 5.00%