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, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Available For Sale Securities [Abstract]
Fixed-maturity securities held as available-for-sale, amortized cost	$ 4,197	$ 4,438
Available For Sale Securities	$ 4,144	$ 4,423
> 5% To 15% [Member]
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%
> 5% To 15% [Member] \| Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position \| security	37
Fixed-maturity securities held as available-for-sale, amortized cost	$ 225
Available For Sale Securities	$ 206
> 15% To 25% [Member]
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%
> 15% To 25% [Member] \| Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position \| security	2
Fixed-maturity securities held as available-for-sale, amortized cost	$ 12
Available For Sale Securities	$ 10
> 25% To 50% [Member]
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%
> 25% To 50% [Member] \| Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position \| security	1
Fixed-maturity securities held as available-for-sale, amortized cost	$ 1
Available For Sale Securities	$ 0
> 50% [Member]
Held To Maturity Securities [Abstract]
Percentage Of Fair Value Below Book Value Minimum	50.00%
> 50% [Member] \| Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position \| security	3
Fixed-maturity securities held as available-for-sale, amortized cost	$ 101
Available For Sale Securities	$ 27
Greater Than 5% [Member] \| Unrealized loss position > 12 months
Available For Sale Securities [Abstract]
Number of available-for-sale securities in unrealized loss position \| security	43
Fixed-maturity securities held as available-for-sale, amortized cost	$ 339
Available For Sale Securities	$ 243

Percentage Of Fair Value Below Book Value Maximum

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Percentage Of Fair Value Below Book Value Minimum

+ References

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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