Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 10,135	$ 9,799
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	432	359
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	27	73
Fair Value	10,540	10,085
Debt Securities \| Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,514	5,555
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	280	252
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	11	26
Fair Value	5,783	5,781
Debt Securities \| States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	4,103	3,770
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	144	100
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13	42
Fair Value	4,234	3,828
Debt Securities \| Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	280	282
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	8	7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	2
Fair Value	288	287
Debt Securities \| Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	207	167
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3	3
Fair Value	204	164
Debt Securities \| United States government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	16	10
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	16	10
Debt Securities \| Foreign government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	10	10
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	10	10
Debt Securities \| Convertibles and bonds with warrants attached
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5	5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	5	5
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,264	2,995
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,799	2,348
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	38	9
Fair Value	6,025	5,334
Equity securities \| Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,084	2,812
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,762	2,320
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	38	9
Fair Value	5,808	5,123
Equity securities \| Nonredeemable preferred equities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	180	183
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	37	28
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	217	211
Fixed Maturities And Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	13,399	12,794
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3,231	2,707
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	65	82
Fair Value	$ 16,565	$ 15,419

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

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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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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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