Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	$ 12,034	$ 11,312
Fixed maturities, at fair value	12,908	12,338
Debt Securities
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	12,034	11,312
Gross Unrealized Gain	886	1,036
Gross Unrealized Loss	12	10
Fixed maturities, at fair value	12,908	12,338
Debt Securities \| Corporate
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	6,962	6,281
Gross Unrealized Gain	542	621
Gross Unrealized Loss	8	7
Fixed maturities, at fair value	7,496	6,895
Debt Securities \| States, municipalities and political subdivisions
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	4,646	4,604
Gross Unrealized Gain	328	395
Gross Unrealized Loss	4	2
Fixed maturities, at fair value	4,970	4,997
Debt Securities \| Commercial mortgage-backed
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	266	271
Gross Unrealized Gain	13	15
Gross Unrealized Loss	0	1
Fixed maturities, at fair value	279	285
Debt Securities \| United States government
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	127	115
Gross Unrealized Gain	3	5
Gross Unrealized Loss	0	0
Fixed maturities, at fair value	130	120
Debt Securities \| Foreign government
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	26	29
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fixed maturities, at fair value	26	29
Debt Securities \| Government-sponsored enterprises
Debt Securities, Available-for-sale
Debt Securities, Available-for-sale, Amortized Cost	7	12
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fixed maturities, at fair value	$ 7	$ 12

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amount of unrealized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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+ 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 (c)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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