Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021
Segment Reconciling Items \| Catastrophe
Prior accident year development [1]	$ (32)	$ (98)
Accident and Health Insurance Product Line [Member] \| Segment Reconciling Items
Prior accident year development [1]	(143)	(113)
Accident and Health Insurance Product Line [Member] \| Segment Reconciling Items \| COVID-19 related Claims
Prior accident year development [1]	14
Change in Workers Compensation Discount Including Accretion [Member] \| Segment Reconciling Items
Prior accident year development [1]	27	26
General Liability [Member] \| Segment Reconciling Items
Prior accident year development [1]	33	451
Marine \| Segment Reconciling Items
Prior accident year development [1]	2	0
Package Business [Member] \| Segment Reconciling Items
Prior accident year development [1]	(35)	(66)
Property Insurance [Member] \| Commercial Lines \| Segment Reconciling Items
Prior accident year development [1]	(17)	(24)
Property Insurance [Member] \| Personal Lines \| Segment Reconciling Items
Prior accident year development [1]	(2)	2
Professional Liability Insurance [Member] \| Segment Reconciling Items
Prior accident year development [1]	(9)	(7)
Surety Product Line [Member] \| Segment Reconciling Items
Prior accident year development [1]	(4)	(26)
Assumed reinsurance \| Segment Reconciling Items
Prior accident year development [1]	12	0
Automobiles [Member] \| Commercial Lines \| Segment Reconciling Items
Prior accident year development [1]	23	9
Automobiles [Member] \| Personal Lines \| Segment Reconciling Items
Prior accident year development [1]	(14)	(73)
Uncollectible reinsurance \| Segment Reconciling Items
Prior accident year development [1]	3	(10)
Insurance, Other [Member] \| Segment Reconciling Items
Prior accident year development [1]	9	26
Property, Liability and Casualty Insurance Product Line
Prior accident year development [1]	(147)	170
Prior accident year development before change in deferred gain	(147)	97
Property, Liability and Casualty Insurance Product Line \| Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Increase (Decrease) in Deferred Revenue	$ 0	$ 73

Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit

No definition available.

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

- Definition

Amount of increase (decrease) in deferred income and obligation to transfer product and service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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