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Investments (Narrative) (Detail)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2025
USD ($)
Security
Mar. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
Security
Schedule Of Investments [Line Items]      
Fair value of securities on deposit with various regulatory authorities $ 11   $ 11
Number of securities in unrealized loss position for a continuous 12 month period | Security 304   366
Rate that a security's fair value is below book value 5.00%   5.00%
Increase decrease in other net realized (gains) losses $ (7) $ (3)  
Asset Pledged as Collateral [Member]      
Schedule Of Investments [Line Items]      
Fair value of securities pledged as collateral $ 212   $ 213
Securities In Unrealized Loss Position [Member]      
Schedule Of Investments [Line Items]      
Weighted average contractual maturity period in years for securities in an unrealized loss position 14 years   15 years
Fair Value Below Book Value Greater Than Five Percent [Member]      
Schedule Of Investments [Line Items]      
Number of securities in unrealized loss position for a continuous 12 month period | Security 244   318