Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Segment Reconciling Items
Increase (Decrease) in Deferred Revenue	$ 6	$ 29
Segment Reconciling Items \| Catastrophe
Prior accident year development [1]	(16)	(13)
Accident and Health Insurance Product Line [Member] \| Segment Reconciling Items
Prior accident year development [1]	(40)	(17)
Change in Workers Compensation Discount Including Accretion [Member] \| Segment Reconciling Items
Prior accident year development [1]	9	9
General Liability [Member] \| Segment Reconciling Items
Prior accident year development [1]	307	12
Marine \| Segment Reconciling Items
Prior accident year development [1]	6	0
Package Business [Member] \| Segment Reconciling Items
Prior accident year development [1]	(27)	1
Property Insurance [Member] \| Commercial Lines \| Segment Reconciling Items
Prior accident year development [1]	(13)	(7)
Property Insurance [Member] \| Personal Lines \| Segment Reconciling Items
Prior accident year development [1]	(3)	(2)
Professional Liability Insurance [Member] \| Segment Reconciling Items
Prior accident year development [1]	(1)	1
Surety Product Line [Member] \| Segment Reconciling Items
Prior accident year development [1]	0	0
Assumed reinsurance \| Segment Reconciling Items
Prior accident year development [1]	2	0
Automobiles [Member] \| Commercial Lines \| Segment Reconciling Items
Prior accident year development [1]	0	5
Automobiles [Member] \| Personal Lines \| Segment Reconciling Items
Prior accident year development [1]	(23)	(6)
Uncollectible reinsurance \| Segment Reconciling Items
Prior accident year development [1]	(9)	0
Insurance, Other [Member] \| Segment Reconciling Items
Prior accident year development [1]	31	11
Property, Liability and Casualty Insurance Product Line
Prior accident year development [1]	229	23
Prior accident year development before change in deferred gain	$ 223	$ (6)

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

No definition available.

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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 http://asc.fasb.org/extlink&oid=121586364&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 http://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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