Reinsurance - Schedule of the Inception Date, Covered Production Period, Contractual and Optional Termination Dates, Current Cession Rate, Ceding Commission and Profit Commission for Each Outstanding QSR Transaction (Details) - Third-Party Reinsurers		12 Months Ended
	Jan. 01, 2026	Dec. 31, 2025
Reinsurance Policy, Type [Axis]: 2020 QSR Transaction \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	45.00%
Reinsurance Policy, Type [Axis]: 2022 QSR Transaction \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	68.00%
Reinsurance Policy, Type [Axis]: Amended Seasoned 2022 QSR Transaction \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	53.00%
Reinsurance Policy, Type [Axis]: 2018 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding Commission		20.00%
Profit Commission		0.61
Reinsurance Policy, Type [Axis]: 2018 QSR Transaction \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	45.00%
Reinsurance Policy, Type [Axis]: 2018 QSR Transaction, Period 1
Ceded Credit Risk [Line Items]
Ceding percentage		21.00%
Reinsurance Policy, Type [Axis]: 2018 QSR Transaction, Period 2
Ceded Credit Risk [Line Items]
Ceding percentage		17.00%
Reinsurance Policy, Type [Axis]: 2020 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		21.00%
Ceding Commission		36.00%
Profit Commission		0.50
Reinsurance Policy, Type [Axis]: 2021 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		20.00%
Ceding Commission		35.00%
Profit Commission		0.54
Reinsurance Policy, Type [Axis]: 2022 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		20.00%
Ceding Commission		20.00%
Profit Commission		0.62
Reinsurance Policy, Type [Axis]: 2022 Seasoned QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		95.00%
Ceding Commission		35.00%
Profit Commission		0.55
Reinsurance Policy, Type [Axis]: 2023 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		20.00%
Ceding Commission		20.00%
Profit Commission		0.62
Reinsurance Policy, Type [Axis]: 2024 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		20.00%
Ceding Commission		20.00%
Profit Commission		0.56
Reinsurance Policy, Type [Axis]: 2025 QSR Transaction
Ceded Credit Risk [Line Items]
Ceding percentage		20.00%
Ceding Commission		20.00%
Profit Commission		0.62
Reinsurance Policy, Type [Axis]: 2026 QSR Transactions
Ceded Credit Risk [Line Items]
Ceding percentage		25.00%
Reinsurance Policy, Type [Axis]: 2027 QSR Transactions
Ceded Credit Risk [Line Items]
Ceding percentage		35.00%
Reinsurance Policy, Type [Axis]: 2028 QSR Transactions
Ceded Credit Risk [Line Items]
Ceding percentage		20.00%
Reinsurance Policy, Type [Axis]: Amended 2018 QSR Transactions \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding percentage	16.00%
Reinsurance Policy, Type [Axis]: Amended 2018 and 2019 QSR Transactions \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	42.00%
Reinsurance Policy, Type [Axis]: Amended 2019 QSR Transactions \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding percentage	13.00%
Reinsurance Policy, Type [Axis]: Amended 2020 QSR Transaction \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	45.00%
Reinsurance Policy, Type [Axis]: Amended Seasoned 2022 QSR Transactions \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding percentage	48.00%
Reinsurance Policy, Type [Axis]: Seasoned 2022 QSR Transaction \| Subsequent Event
Ceded Credit Risk [Line Items]
Ceding Commission	38.00%

NMI Holdings, Inc. 10-K