Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Change in deferred gain on retroactive reinsurance included in other liabilities [1]	$ 29	$ 0
Camp Wildfire [Member]
Gain Contingencies [Line Items]
Salvage and Subrogation Recoveries, Value	57
Unrecorded estimated litigation settlement, amount due to reinsurer	57
Per occurrence retention [Member] \| Camp Wildfire [Member]
Gain Contingencies [Line Items]
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments	12
Reinsurance Retention Policy, Amount Retained	350
Aggregate excess of loss treaty [Member] \| Camp Wildfire [Member]
Gain Contingencies [Line Items]
Reinsurance Recoverable for Paid and Unpaid Claims and Claims Adjustments	45
Reinsurance Retention Policy, Amount Retained	825
PG&E [Member] \| Portion of Expected Class Settlement Due to HIG [Member] \| Minimum
Gain Contingencies [Line Items]
Gain Contingency, Unrecorded Amount	300
PG&E [Member] \| Portion of Expected Class Settlement Due to HIG [Member] \| Maximum
Gain Contingencies [Line Items]
Gain Contingency, Unrecorded Amount	325
Workers’ compensation
Prior accident year development [1]	(17)	(20)
Workers’ compensation discount accretion
Prior accident year development [1]	9	8
General Liability [Member]
Prior accident year development [1]	12	6
Marine
Prior accident year development [1]	0	0
Package business
Prior accident year development [1]	1	5
Property
Prior accident year development [1]	(2)	1
Property \| Property and Casualty, Commercial Insurance [Member]
Prior accident year development [1]	(7)	(2)
Professional liability
Prior accident year development [1]	1	0
Bond
Prior accident year development [1]	0	0
Assumed reinsurance
Prior accident year development [1]	0	0
Automobile \| Property and Casualty, Commercial Insurance [Member]
Prior accident year development [1]	5	0
Automobile \| Property and Casualty, Personal Insurance [Member]
Prior accident year development [1]	(6)	(5)
Net asbestos reserves
Prior accident year development [1]	0	0
Net environmental reserves
Prior accident year development [1]	0	0
Catastrophes [Member]
Prior accident year development [1]	(13)	(8)
Uncollectible reinsurance
Prior accident year development [1]	0	0
Other reserve re-estimates, net
Prior accident year development [1]	11	4
Property, Liability and Casualty Insurance Product Line
Prior accident year development [1]	23	(11)
Prior accident year development before change in deferred gain	$ (6)	$ (11)

Change in Deferred Gain on Retroactive Reinsurance

No definition available.

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Unrecorded estimated litigation settlement, amount due to reinsurer

+ References

No definition available.

+ Details

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- 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

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- Definition

The amount or range of possible amounts of gain that could be realized upon the resolution of a contingency.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 30
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6397426&loc=d3e17499-108355

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- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=99380617&loc=SL75241803-196195

+ Details

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Period Type:	instant

- Definition

Stated amount that is the upper retention limit per entity policy.

+ References

No definition available.

+ Details

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- Definition

The estimated amount as of the balance sheet date to be received by an insurer from the sale of property (usually damaged) on which the insurer has paid a total claim to the insured and has obtained title to the property and the right of an insurer to pursue any course of recovery of damages, in its name or in the name of the policyholder, against a third party who is liable for costs relating to an insured event that have been paid by the insurer. Estimated salvage and subrogation is generally deducted from the liability for unpaid claims.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 30
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13783-158432

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- 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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