Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - Debt Securities - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Cost or Amortized Cost	$ 11,273	$ 11,108
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	916	597
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	32	7
Fair Value	12,157	11,698
Corporate
Debt Securities, Available-for-sale
Cost or Amortized Cost	6,264	6,074
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	532	332
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	27	5
Fair Value	6,769	6,401
States, municipalities and political subdivisions
Debt Securities, Available-for-sale
Cost or Amortized Cost	4,579	4,477
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	366	252
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3	1
Fair Value	4,942	4,728
Commercial mortgage-backed
Debt Securities, Available-for-sale
Cost or Amortized Cost	271	290
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	13	11
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2	0
Fair Value	282	301
United States government
Debt Securities, Available-for-sale
Cost or Amortized Cost	119	102
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	5	2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	124	104
Foreign government
Debt Securities, Available-for-sale
Cost or Amortized Cost	27	28
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	27	28
Government-sponsored enterprises
Debt Securities, Available-for-sale
Cost or Amortized Cost	13	137
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	1
Fair Value	$ 13	$ 136

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

No definition available.

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: