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) $ in Millions	Sep. 30, 2025 USD ($) Security	Dec. 31, 2024 USD ($)
Available For Sale Securities [Abstract]
Book Value	$ 1,521
Fair Value	$ 1,401	$ 1,236
> 5% To 15% [Member] \| Unrealized loss position > 12 months [Member]
Available For Sale Securities [Abstract]
Number of securities \| Security	90
Book Value	$ 286
Fair Value	$ 256
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] \| Unrealized loss position > 12 months [Member]
Available For Sale Securities [Abstract]
Number of securities \| Security	49
Book Value	$ 81
Fair Value	$ 65
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] \| Unrealized loss position > 12 months [Member]
Available For Sale Securities [Abstract]
Number of securities \| Security	55
Book Value	$ 214
Fair Value	$ 137
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] \| Unrealized loss position > 12 months [Member]
Available For Sale Securities [Abstract]
Number of securities \| Security	2
Book Value	$ 0
Fair Value	$ 0
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	50.00%
Greater Than 5% [Member] \| Unrealized loss position > 12 months [Member]
Available For Sale Securities [Abstract]
Number of securities \| Security	196
Book Value	$ 581
Fair Value	$ 458

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