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	Mar. 31, 2024 USD ($) Security	Dec. 31, 2023 USD ($)
Available For Sale Securities [Abstract]
Book Value	$ 1,544
Fair Value	$ 1,397	$ 1,557
> 5% To 15% [Member] \| Unrealized loss position > 12 months [Member]
Available For Sale Securities [Abstract]
Number of securities \| Security	176
Book Value	$ 231
Fair Value	$ 208
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	108
Book Value	$ 277
Fair Value	$ 226
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	79
Book Value	$ 227
Fair Value	$ 152
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	365
Book Value	$ 735
Fair Value	$ 586

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
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-Section 50
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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
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