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Reinsurance - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Policyholder Benefits and Claims Incurred, Ceded	$ 300
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group | Remaining [Member]
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	0
Reinsurance Policy, Type [Axis]: Asbestos and Environmental | Asbestos and Environmental
Policyholder Benefits and Claims Incurred, Ceded	1,500
Reinsurance Policy, Type [Axis]: Asbestos and Environmental | Remaining [Member] | Asbestos and Environmental
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	0
Property and Casualty Insurance Products
Policyholder Benefits and Claims Incurred, Ceded	1,241	$ 1,043	$ 1,338
Group Insurance Policy [Member]
Policyholder Benefits and Claims Incurred, Ceded	120	$ 93	$ 81
Maximum | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	300
Maximum | Reinsurance Policy, Type [Axis]: Asbestos and Environmental | Asbestos and Environmental
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	1,500
Maximum | The Navigators Group, Inc. [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	$ 300

X

- Definition

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479838/944-20-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

+ Details

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X

- Definition

Amount per policy of risk ceded to reinsurer in excess of risk undertaken originally by insurer that is not ceded to reinsurer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479838/944-20-50-3

+ Details

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