Description of Business - Additional Information (Details) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
Sep. 30, 2025 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2025 USD ($) Segment	Dec. 31, 2024 USD ($)
Business Overview [Abstract]
Number of reportable business segments \| Segment			1
Expected proceeds from intercompany notes borrowings	$ 600	$ 600	$ 600
Debt instrument term		10 years
Inigo Limited
Business Overview [Abstract]
Business acquisition, agreement to acquire for purchase price in cash transaction	$ 1,700
Mortgage Insurance revenues
Business Overview [Abstract]
Down payment purchase price (percent)	20.00%	20.00%	20.00%
Equity in home (percent)	20.00%	20.00%	20.00%
Direct primary mortgage insurance in force	$ 280,600	$ 280,600	$ 280,600	$ 275,100
Direct primary mortgage insurance risk in force	$ 74,000	$ 74,000	$ 74,000	$ 72,100

Business Overview [Abstract]

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Expected proceeds from intercompany notes borrowings.

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- Definition

This element represents the maximum down payment percentage on a home's purchase price that is used to measure the need for private mortgage insurance.

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This element represents the maximum equity-ownership percentage on a home's value that is used to measure the need for private mortgage insurance for refinancings.

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Risk In Force, Ceded Amount

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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- Definition

Amount of participating insurance in force.

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- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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