Investments (Narrative) (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024 USD ($) Security	Mar. 31, 2023 USD ($)	Dec. 31, 2023 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	433		450
Rate that a security's fair value is below book value	5.00%		5.00%
Increase decrease in other net realized (gains) losses	$ (2)	$ 15
Asset Pledged as Collateral [Member]
Schedule Of Investments [Line Items]
Fair value of securities pledged as collateral	$ 230		$ 241
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		14 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	365		365