Premiums Receivable - Rollforward of Premiums Receivable and Agents' Balances (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss	$ (134)	$ (183)	$ (152)	$ (145)
Premium Receivable, Credit Loss Expense (Reversal)	1	2	8	83
Premium Receivable, Allowance for Credit Loss, Writeoff	(13)	(12)	(43)	(35)
Premium Receivable, Allowance for Credit Loss, Recovery	2	1	7	4
Premium Receivable, Allowance for Credit Loss	(124)	(174)	(124)	(174)
Premiums Receivable, Due in One Year or Less, Excluding Receivables for Loss within a Deductible and Retrospectively-Rated Policy Premiums [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss	(97)	(139)	(117)	(85)
Premium Receivable, Credit Loss Expense (Reversal)	1	2	6	78
Premium Receivable, Allowance for Credit Loss, Writeoff	(13)	(12)	(43)	(35)
Premium Receivable, Allowance for Credit Loss, Recovery	2	1	7	4
Premium Receivable, Allowance for Credit Loss	(87)	(130)	(87)	(130)
Premiums Receivable, Loss Within a Deductible and Retrospectively-Rated Policy Premiums [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss	(37)	(44)	(35)	(60)
Premium Receivable, Credit Loss Expense (Reversal)	0	0	2	5
Premium Receivable, Allowance for Credit Loss, Writeoff	0	0	0	0
Premium Receivable, Allowance for Credit Loss, Recovery	0	0	0	0
Premium Receivable, Allowance for Credit Loss	$ (37)	$ (44)	$ (37)	(44)
Cumulative Effect, Period of Adoption, Adjustment [Member] \| Accounting Standards Update 2016-13 [Member] \| Premiums Receivable [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss				23
Cumulative Effect, Period of Adoption, Adjustment [Member] \| Premiums Receivable, Due in One Year or Less, Excluding Receivables for Loss within a Deductible and Retrospectively-Rated Policy Premiums [Member] \| Accounting Standards Update 2016-13 [Member] \| Premiums Receivable [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss				2
Cumulative Effect, Period of Adoption, Adjustment [Member] \| Premiums Receivable, Loss Within a Deductible and Retrospectively-Rated Policy Premiums [Member] \| Accounting Standards Update 2016-13 [Member] \| Premiums Receivable [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss				21
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss				(122)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] \| Premiums Receivable, Due in One Year or Less, Excluding Receivables for Loss within a Deductible and Retrospectively-Rated Policy Premiums [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss				(83)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] \| Premiums Receivable, Loss Within a Deductible and Retrospectively-Rated Policy Premiums [Member]
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Premium Receivable, Allowance for Credit Loss				$ (39)

Amount of expense (reversal of expense) for expected credit loss on premium receivable.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(5))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in allowance for credit loss on premium receivable from recovery.

+ References

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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- Definition

Amount of writeoff of premium receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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- Details

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