XML 54 R42.htm IDEA: XBRL DOCUMENT
v3.25.3

Reinsurance - Narrative (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
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 |  |  |  |

X
- Definition Amount of anticipated payments related to operating expenses of counterparty. + References No definition available. + Details
Name: | nmih_AnticipatedOperatingExpensePaymentsAnnual
Namespace Prefix: | nmih_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of Additional Sequential Quota Share Reinsurance Treaties + References No definition available. + Details
Name: | nmih_NumberOfAdditionalSequentialQuotaShareReinsuranceTreaties
Namespace Prefix: | nmih_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Excess-of-Loss Reinsurance Agreements + References No definition available. + Details
Name: | nmih_NumberOfExcessOfLossReinsuranceAgreements
Namespace Prefix: | nmih_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Quota Share Reinsurance Treaties + References No definition available. + Details
Name: | nmih_NumberOfQuotaShareReinsuranceTreaties
Namespace Prefix: | nmih_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Premiums Earned, Percent Of Risk + References No definition available. + Details
Name: | nmih_PremiumsEarnedPercentOfRisk
Namespace Prefix: | nmih_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reinsurance Agreement, Optional Termination Right, Percent Of Reinsurance Coverage Threshold + References No definition available. + Details
Name: | nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold
Namespace Prefix: | nmih_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Reinsurance Retention Policy, Ceded Aggregate Premiums + References No definition available. + Details
Name: | nmih_ReinsuranceRetentionPolicyCededAggregatePremiums
Namespace Prefix: | nmih_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Reinsurance Retention Policy Excess Of Loss Reinsurance Coverages + References No definition available. + Details
Name: | nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages
Namespace Prefix: | nmih_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Reinsurance Retention Policy, Term Of Underlying Mortgage Amortization + References No definition available. + Details
Name: | nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization
Namespace Prefix: | nmih_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reinsurance Retention, Risk Premiums Paid + References No definition available. + Details
Name: | nmih_ReinsuranceRetentionRiskPremiumsPaid
Namespace Prefix: | nmih_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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/2003/role/disclosureRef -Topic 944 -SubTopic 825 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477351/944-825-50-1B Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 825 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477351/944-825-50-1B + Details
Name: | us-gaap_CededCreditRiskLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after valuation allowance, recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479838/944-20-50-5 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477363/944-310-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 45 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477409/944-310-45-6 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-11 Reference 5: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-15 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 825 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477351/944-825-50-1B Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 45 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477409/944-310-45-5 + Details
Name: | us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482913/230-10-50-8 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(1)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
Name: | us-gaap_RestrictedCash
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | srt_CounterpartyNameAxis=nmih_A2026XOLTransactionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CounterpartyNameAxis=nmih_A20262XOLTransactionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2016 QSR Transactions
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=QSR Transactions
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CededCreditRiskAxis=nmih_ThirdPartyReinsurersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2018 QSR Transaction, Period 1
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2018 QSR Transaction, Period 2
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2020 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2021 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2022 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2022 Seasoned QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2023 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2024 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2025 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2026 QSR Transaction and 2027 QSR Transaction
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2026 QSR Transactions
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=2027 QSR Transactions
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ReinsuranceContractAxis=Oaktown Re Vehicles
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |