Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - Debt Securities - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 10,832	$ 10,643
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	502	174
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	14	128
Fair Value	11,320	10,689
Corporate
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	5,815	5,712
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	269	85
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13	87
Fair Value	6,071	5,710
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	4,331	4,251
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	218	84
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	31
Fair Value	4,549	4,304
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	289	287
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	12	3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	2
Fair Value	301	288
Government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	283	316
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1	7
Fair Value	282	310
United States government
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	97	67
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	1
Fair Value	100	67
Foreign government
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	17	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	$ 17	$ 10

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

No definition available.

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
-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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- Definition

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

No definition available.

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- Definition

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

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- Definition

Line items represent financial 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.

+ References

No definition available.

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