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Regulatory Information - Schedule of Statutory Net Income (loss), Surplus, Contingency Reserve and Risk-to-Capital Ratio (Details) - NMIC and Re One combined $ in Thousands	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Statutory Accounting Practices [Line Items]
Statutory surplus	$ 982,245	$ 984,362
Contingency reserve	2,161,465	1,905,990
Statutory capital	$ 3,143,710	$ 2,890,352
Risk-to-capital	13.1	12.7

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- Definition

This concept represents an entity's ratio of net risk in force (excludes risk ceded through reinsurance contracts and risk in force on defaulted loans) to the amount of statutory capital (defined as the sum of statutory policyholders' surplus plus statutory contingency reserves).

+ References

No definition available.

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- Definition

Contingency reserve recorded under Statutory Accounting Principles is a liability to protect against loss during periods of extreme contraction.

+ References

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- Definition

Statutory Accounting Practices, Statutory Capital and Surplus Including Contingency Reserve

+ References

No definition available.

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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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
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- Definition

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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