Reinsurance - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
Reinsurance - Narrative (Details) $ in Thousands	Oct. 31, 2024 USD ($)	Sep. 30, 2025 USD ($) reinsuranceAgreement quota_share_agreement	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) reinsuranceAgreement quota_share_agreement	Sep. 30, 2024 USD ($)	Jan. 01, 2025 claim	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Ceded Credit Risk [Line Items]
Number of quota share reinsurance treaties \| quota_share_agreement		8		8
Reinsurance recoverable on unpaid claims		$ 35,315	$ 29,214	$ 35,315	$ 29,214		$ 32,260	$ 27,514
Number of excess-of-loss reinsurance agreements \| reinsuranceAgreement		7		7
Reinsurance coverage amount		$ 10,700	9,800	$ 31,200	28,400
Reinsurance coverage amount amortized (in percent)		10.00%		10.00%
Restricted cash		$ 0		$ 0			90
2026 XOL Transaction
Ceded Credit Risk [Line Items]
Reinsurance retention policy excess of loss reinsurance coverages	$ 164,200
2026-2 XOL Transaction
Ceded Credit Risk [Line Items]
Reinsurance retention policy excess of loss reinsurance coverages				$ 160,000
Minimum
Ceded Credit Risk [Line Items]
Reinsurance coverage, term of underlying mortgage amortization (in years)				10 years
Maximum
Ceded Credit Risk [Line Items]
Reinsurance coverage, term of underlying mortgage amortization (in years)				12 years 6 months
Reinsurance Policy, Type [Axis]: 2016 QSR Transactions
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid claims							1,100
Reinsurance Policy, Type [Axis]: QSR Transactions
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid claims		35,300		$ 35,300			$ 31,200
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2018 QSR Transaction, Period 1
Ceded Credit Risk [Line Items]
Ceding Percentage				21.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2018 QSR Transaction, Period 2
Ceded Credit Risk [Line Items]
Ceding Percentage				17.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2020 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				21.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2021 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				20.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2022 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				20.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2022 Seasoned QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				95.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2023 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				20.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2024 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				20.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2025 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Percentage				20.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2026 QSR Transaction and 2027 QSR Transaction
Ceded Credit Risk [Line Items]
Number of additional sequential quota share reinsurance treaties \| claim						2
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2026 QSR Transactions
Ceded Credit Risk [Line Items]
Ceding Percentage				20.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: 2027 QSR Transactions
Ceded Credit Risk [Line Items]
Ceding Percentage				12.00%
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: Oaktown Re Vehicles
Ceded Credit Risk [Line Items]
Risk premiums paid		$ 3,000	$ 4,300	$ 9,600	$ 16,100
Third-party Reinsurers \| Reinsurance Policy, Type [Axis]: Oaktown Re Vehicles \| Maximum
Ceded Credit Risk [Line Items]
Anticipated payment related to annual operating expenses				$ 250

NMI Holdings, Inc. 10-Q