Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available For Sale Securities [Abstract]
Less than 12 months, fair value	$ 2,108
Less than 12 months, unrealized losses	(33)
12 months or longer, fair value	580
12 months or longer, unrealized losses	(105)
Total available-for-sale, fair value	2,688
Total available-for-sale, unrealized losses	(138)
Fixed Maturities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	2,108	$ 1,204
Less than 12 months, unrealized losses	(33)	(13)
12 months or longer, fair value	580	617
12 months or longer, unrealized losses	(105)	(93)
Total available-for-sale, fair value	2,688	1,821
Total available-for-sale, unrealized losses	(138)	(106)
Fixed Maturities [Member] \| U S Treasury And Government [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	456	353
Less than 12 months, unrealized losses	(6)	(1)
12 months or longer, fair value	165	124
12 months or longer, unrealized losses	(7)	(5)
Total available-for-sale, fair value	621	477
Total available-for-sale, unrealized losses	(13)	(6)
Fixed Maturities [Member] \| US States And Political Subdivisions [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	190	203
Less than 12 months, unrealized losses	(3)	(8)
12 months or longer, fair value	99	116
12 months or longer, unrealized losses	(6)	(4)
Total available-for-sale, fair value	289	319
Total available-for-sale, unrealized losses	(9)	(12)
Fixed Maturities [Member] \| Foreign Government Debt [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	4	8
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	0
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	4	8
Total available-for-sale, unrealized losses	0	0
Fixed Maturities [Member] \| Corporate Obligations [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	1,139	425
Less than 12 months, unrealized losses	(20)	(3)
12 months or longer, fair value	156	163
12 months or longer, unrealized losses	(84)	(77)
Total available-for-sale, fair value	1,295	588
Total available-for-sale, unrealized losses	(104)	(80)
Fixed Maturities [Member] \| Residential Mortgage Backed Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	144	105
Less than 12 months, unrealized losses	(2)	(1)
12 months or longer, fair value	117	156
12 months or longer, unrealized losses	(5)	(3)
Total available-for-sale, fair value	261	261
Total available-for-sale, unrealized losses	(7)	(4)
Fixed Maturities [Member] \| Residential Mortgage Backed Non Agency [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	0	0
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	16	14
12 months or longer, unrealized losses	(3)	(4)
Total available-for-sale, fair value	16	14
Total available-for-sale, unrealized losses	(3)	(4)
Fixed Maturities [Member] \| Commercial Mortgage Backed Securities [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	30	27
Less than 12 months, unrealized losses	(1)	0
12 months or longer, fair value	4	5
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	34	32
Total available-for-sale, unrealized losses	(1)	0
Fixed Maturities [Member] \| Collateralized Debt Obligations [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	9	12
Less than 12 months, unrealized losses	0	0
12 months or longer, fair value	0	0
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	9	12
Total available-for-sale, unrealized losses	0	0
Fixed Maturities [Member] \| Other Asset Backed [Member]
Available For Sale Securities [Abstract]
Less than 12 months, fair value	136	71
Less than 12 months, unrealized losses	(1)	0
12 months or longer, fair value	23	39
12 months or longer, unrealized losses	0	0
Total available-for-sale, fair value	159	110
Total available-for-sale, unrealized losses	$ (1)	$ 0

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ 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 (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 320
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-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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