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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 | Dec. 31, 2017 USD ($) security | Dec. 31, 2016 USD ($)
--- | --- | ---
Available For Sale Securities [Abstract] |  |
Fixed-maturity securities held as available-for-sale, amortized cost | $ 4,438 | $ 5,450
Available For Sale Securities | $ 4,423 | $ 5,431
> 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 | 17 |
Fixed-maturity securities held as available-for-sale, amortized cost | $ 76 |
Available For Sale Securities | $ 71 |
Held To Maturity Securities [Abstract] |  |
Number of Held-To-Maturity Securities in unrealized loss position | security | 0 |
Held-To-Maturity Securities Amortized Cost | $ 0 |
Held-To-Maturity Securities Fair Value | $ 0 |
> 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 | 1 |
Fixed-maturity securities held as available-for-sale, amortized cost | $ 3 |
Available For Sale Securities | $ 2 |
Held To Maturity Securities [Abstract] |  |
Number of Held-To-Maturity Securities in unrealized loss position | security | 0 |
Held-To-Maturity Securities Amortized Cost | $ 0 |
Held-To-Maturity Securities Fair Value | $ 0 |
> 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 | 2 |
Fixed-maturity securities held as available-for-sale, amortized cost | $ 12 |
Available For Sale Securities | $ 9 |
Held To Maturity Securities [Abstract] |  |
Number of Held-To-Maturity Securities in unrealized loss position | security | 0 |
Held-To-Maturity Securities Amortized Cost | $ 0 |
Held-To-Maturity Securities Fair Value | $ 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 | 4 |
Fixed-maturity securities held as available-for-sale, amortized cost | $ 101 |
Available For Sale Securities | $ 29 |
Held To Maturity Securities [Abstract] |  |
Number of Held-To-Maturity Securities in unrealized loss position | security | 0 |
Held-To-Maturity Securities Amortized Cost | $ 0 |
Held-To-Maturity Securities Fair Value | $ 0 |
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 | 24 |
Fixed-maturity securities held as available-for-sale, amortized cost | $ 192 |
Available For Sale Securities | $ 111 |
Held To Maturity Securities [Abstract] |  |
Number of Held-To-Maturity Securities in unrealized loss position | security | 0 |
Held-To-Maturity Securities Amortized Cost | $ 0 |
Held-To-Maturity Securities Fair Value | $ 0 |

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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=65884222&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=73527792&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=66010992&loc=d3e26610-111562 + Details
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- Definition 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 + Details
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- Definition 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=73527792&loc=d3e27290-111563 + Details
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