Investments (Distribution Of Securities By Percentage Of Fair Value Below Book Value By More Than 5% For A Continuous Twelve Month Period Or Longer) (Detail) - Unrealized loss position > 12 months [Member] $ in Millions	Sep. 30, 2023 USD ($) Security
> 5% To 15% [Member]
Available For Sale Securities [Abstract]
Number of securities | Security	194
Book Value	$ 336
Fair Value	$ 304
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	5.00%
Percentage Of Fair Value Below Book Value Maximum	15.00%
> 15% To 25% [Member]
Available For Sale Securities [Abstract]
Number of securities | Security	153
Book Value	$ 386
Fair Value	$ 309
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	15.00%
Percentage Of Fair Value Below Book Value Maximum	25.00%
> 25% To 50% [Member]
Available For Sale Securities [Abstract]
Number of securities | Security	159
Book Value	$ 373
Fair Value	$ 244
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	25.00%
Percentage Of Fair Value Below Book Value Maximum	50.00%
> 50% [Member]
Available For Sale Securities [Abstract]
Number of securities | Security	3
Book Value	$ 5
Fair Value	$ 2
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	50.00%
Greater Than 5% [Member]
Available For Sale Securities [Abstract]
Number of securities | Security	509
Book Value	$ 1,100
Fair Value	$ 859