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Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 12 Months Ended
Jan. 31, 2025
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Dec. 31, 2024
Defined Benefit Plan Disclosure [Line Items]          
Proceeds from issuance of common stock   $ 55 $ 1,335    
Money Market Accounts          
Defined Benefit Plan Disclosure [Line Items]          
Total cash and cash equivalents, restricted cash and fiduciary cash   3,272 17,045    
Proceeds from AssuredPartners financing   13,500      
Money Market Accounts | Interest income, premium finance revenues and other income (loss)          
Defined Benefit Plan Disclosure [Line Items]          
Increases (decrease) dividend income   (161)      
Dividend income   $ 86 247    
Money Market Accounts | Over-Allotment Option          
Defined Benefit Plan Disclosure [Line Items]          
Proceeds from issuance of common stock $ 1,300        
Qualified Contributory Savings and Thrift 401(k) Plan          
Defined Benefit Plan Disclosure [Line Items]          
Matching contributions by employer, percentage   100.00%      
Percentage of eligible compensation for matching contributions by employer   5.00%      
Matching contributions vesting schedule   5 years      
Contribution expense to plan   $ 40 $ 30    
Employer match on eligible compensation       5.00% 5.00%