Investments (Schedule Of Amortized Cost And Fair Value Of HTM Fixed Maturities And Related OTTI Recognized In Accumulated Other Comprehensive Income) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Held to maturity, Amortized Cost	$ 14,253	$ 14,335
Held to maturity, Gross Unrealized Appreciation	81	179
Held to maturity, Gross Unrealized Depreciation	(212)	(40)
Held to maturity, at Fair Value	14,122	14,474
Held to maturity, OTTI recognized in AOCI		0
U.S. Treasury And Agency [Member]
Investment [Line Items]
Held to maturity, Amortized Cost	1,037	908
Held to maturity, Gross Unrealized Appreciation	9	12
Held to maturity, Gross Unrealized Depreciation	(14)	(5)
Held to maturity, at Fair Value	1,032	915
Held to maturity, OTTI recognized in AOCI	0	0
Foreign [Member]
Investment [Line Items]
Held to maturity, Amortized Cost	1,754	1,738
Held to maturity, Gross Unrealized Appreciation	17	27
Held to maturity, Gross Unrealized Depreciation	(20)	(8)
Held to maturity, at Fair Value	1,751	1,757
Held to maturity, OTTI recognized in AOCI	0	0
Corporate [Member]
Investment [Line Items]
Held to maturity, Amortized Cost	3,026	3,159
Held to maturity, Gross Unrealized Appreciation	26	67
Held to maturity, Gross Unrealized Depreciation	(55)	(7)
Held to maturity, at Fair Value	2,997	3,219
Held to maturity, OTTI recognized in AOCI	0	0
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Held to maturity, Amortized Cost	2,681	2,724
Held to maturity, Gross Unrealized Appreciation	11	23
Held to maturity, Gross Unrealized Depreciation	(45)	(5)
Held to maturity, at Fair Value	2,647	2,742
Held to maturity, OTTI recognized in AOCI	0	0
States, Municipalities, And Political Subdivisions [Member]
Investment [Line Items]
Held to maturity, Amortized Cost	5,755	5,806
Held to maturity, Gross Unrealized Appreciation	18	50
Held to maturity, Gross Unrealized Depreciation	(78)	(15)
Held to maturity, at Fair Value	5,695	5,841
Held to maturity, OTTI recognized in AOCI	$ 0	$ 0

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as held-to-maturity that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 9A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284427-111562

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
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-Section S99
-Paragraph 1
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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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