Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - Debt Securities - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Cost or Amortized Cost	$ 11,478	$ 11,312
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	849	1,036
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	19	10
Fair Value	12,308	12,338
Corporate
Debt Securities, Available-for-sale
Cost or Amortized Cost	6,426	6,281
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	505	621
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	11	7
Fair Value	6,920	6,895
States, municipalities and political subdivisions
Debt Securities, Available-for-sale
Cost or Amortized Cost	4,641	4,604
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	326	395
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	7	2
Fair Value	4,960	4,997
Commercial mortgage-backed
Debt Securities, Available-for-sale
Cost or Amortized Cost	272	271
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	14	15
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1	1
Fair Value	285	285
United States government
Debt Securities, Available-for-sale
Cost or Amortized Cost	109	115
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	4	5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	113	120
Foreign government
Debt Securities, Available-for-sale
Cost or Amortized Cost	24	29
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	24	29
Government-sponsored enterprises
Debt Securities, Available-for-sale
Cost or Amortized Cost	6	12
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	$ 6	$ 12

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=121645371&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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