Liability for Paid Claims and Claims Adjustment Expense, Number of Covered Claims Included In Number of Paid Claims

No definition available.

Name:	nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims
Namespace Prefix:	nmih_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Liability For Unpaid Claims And Claims Adjustment Expense, Incurred But Not Reported (IBNR) Claims, Number Of Primary Loans In Default

+ References

No definition available.

+ Details

Name:	nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault
Namespace Prefix:	nmih_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Liability For Unpaid Claims And Claims Adjustment Expense, Incurred But Not Reported (IBNR) Claims, Number Of Primary Loans In Default, Percent Of Total Policies In-Force

+ References

No definition available.

+ Details

Name:	nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce
Namespace Prefix:	nmih_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Liability For Unpaid Claims And Claims Adjustment Expense, Incurred But Not Reported (IBNR) Claims, Total Policies In-Force

+ References

No definition available.

+ Details

Name:	nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce
Namespace Prefix:	nmih_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Liability for Paid Claims and Claims Adjustment Expense, Number of Paid Claims

+ References

No definition available.

+ Details

Name:	nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims
Namespace Prefix:	nmih_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Liability for Prior Year Claims and Claims Adjustment Expense

+ References

No definition available.

+ Details

Name:	nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense
Namespace Prefix:	nmih_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of payments to settle insured claims and pay costs incurred in the claims settlement process, including amount covered by insurance.

+ References

No definition available.

+ Details

Name:	nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance
Namespace Prefix:	nmih_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Component of total amount of payments to settle insured claims and pay costs incurred in the claims settlement process, including amount covered by insurance, covered under QSR transaction.

+ References

No definition available.

+ Details

Name:	nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction
Namespace Prefix:	nmih_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Prior Year Claims and Claims Adjustment Expense, Favorable Development on Prior Year Defaults

+ References

No definition available.

+ Details

Name:	nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults
Namespace Prefix:	nmih_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-7

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (cc)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-3

Reference 3: 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 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 9E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E

Reference 5: 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)(13)(a)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

+ Details

Name:	us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (cc)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-3

+ Details

Name:	us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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

Reserves for Insurance Claims and Claim Expenses - Narrative (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2026 USD ($) claim policy loan	Jun. 30, 2025 USD ($)	Dec. 31, 2025 USD ($) loan policy	Dec. 31, 2024 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Total gross liability for unpaid claims and claim adjustment expenses	$ 214,583	$ 163,033	$ 196,429	$ 152,071
Number of claims paid \| claim	353
Claims paid, including amounts covered by insurance	$ 23,700
Primary loans in default \| loan	8,020		7,661
Primary insured portfolio, loan default rate (in percent)	1.16%		1.12%
Total number of policies in-force \| policy	694,273		684,058
Favorable development on prior year defaults	$ 40,600	$ 43,800
Reserve for prior year insurance claims and claim expenses	$ 122,600
Reinsurance Policy, Type [Axis]: QSR Transactions
Liability for Claims and Claims Adjustment Expense [Line Items]
Number of covered claims included in number of claims paid \| claim \| claim	340
Component of claims paid covered under QSR transaction	$ 4,500