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Outstanding Exposure - Additional Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2025 USD ($) Category	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of surveillance categories | Category	3
Loss mitigation securities	$ 801	$ 1,200
Aircraft residual value insurance (RVI)
Schedule of Insured Financial Obligations [Line Items]
Residual value insurance policies exposure downgraded	147	147
Net Exposure	87	87
Specialty business, contingent, gross par	60
Aircraft residual value insurance (RVI) | BIG
Schedule of Insured Financial Obligations [Line Items]
Residual value insurance policies exposure downgraded	5	5
Net Exposure	5	$ 5
Commitment to Provide Guarantees | Public finance
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties	1,600
Potential future commitment			$ 353
Commitment to Provide Guarantees | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties	$ 2,200
Potential future commitment			$ 50
Minimum
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)	50.00%

X

- Definition

Aggregate probability weight of possible losses, on a present value basis, for below-investment grade exposures.

+ References

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+ Details

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X

- Definition

Represents the outstanding principal amounts of the securities insured and purchased by the Company, and for which it has expected losses to be paid, in order to mitigate the economic effect of insured losses ("loss mitigation securities").

+ References

No definition available.

+ Details

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- Definition

Number Of Surveillance Categories

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- Definition

Potential Future Commitment

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- Definition

[Line Items] for Schedule of Insured Financial Obligations [Table]

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- Definition

Specialty Business, Contingent, Gross Par

+ References

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- Definition

Specialty Business, Gross Par

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- Definition

Specialty Business, Net Par

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- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477314/942-235-S99-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3

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