Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 10,653	$ 10,314
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	150	409
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	143	24
Fair Value	10,660	10,699
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		3,094
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		3,169
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		14
Fair Value		6,249
Fixed Maturities And Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		13,408
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		3,578
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		38
Fair Value		16,948
Corporate \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,709	5,420
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	94	246
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	60	13
Fair Value	5,743	5,653
States, municipalities and political subdivisions \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	4,306	4,316
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	54	155
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	65	6
Fair Value	4,295	4,465
Commercial mortgage-backed \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	287	280
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2	7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4	1
Fair Value	285	286
Government-sponsored enterprises \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	293	257
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13	4
Fair Value	280	254
United States government \| Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	48	31
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1	0
Fair Value	47	31
Foreign government \| Debt Securities
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
Common equities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		2,918
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		3,135
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		14
Fair Value		6,039
Nonredeemable preferred equities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		176
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		34
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0
Fair Value		$ 210

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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