Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 10,552	$ 10,314
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	190	409
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	106	24
Fair Value	10,636	10,699
Debt Securities \| Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,625	5,420
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	110	246
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	57	13
Fair Value	5,678	5,653
Debt Securities \| States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	4,316	4,316
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	78	155
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	36	6
Fair Value	4,358	4,465
Debt Securities \| Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	283	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	281	286
Debt Securities \| Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	277	257
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8	4
Fair Value	269	254
Debt Securities \| United States government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	41	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	40	31
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
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
Equity securities \| Common equities
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
Equity securities \| Nonredeemable preferred equities
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
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

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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