Amount of anticipated payments related to operating expenses of counterparty.

No definition available.

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

- Definition

Ceded Credit Risk, Commission Earned, Percentage

+ References

No definition available.

+ Details

Name:	nmih_CededCreditRiskCommissionEarnedPercentage
Namespace Prefix:	nmih_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Ceded Credit Risk, Profit Commission, Loss Ratio On Loans Threshold

+ References

No definition available.

+ Details

Name:	nmih_CededCreditRiskProfitCommissionLossRatioOnLoansThreshold
Namespace Prefix:	nmih_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Premiums Earned, Percent Of Risk

+ References

No definition available.

+ Details

Name:	nmih_PremiumsEarnedPercentOfRisk
Namespace Prefix:	nmih_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Previously Ceded Primary Risk-In-Force Recaptured

+ References

No definition available.

+ Details

Name:	nmih_PreviouslyCededPrimaryRiskInForceRecaptured
Namespace Prefix:	nmih_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reinsurance Agreement, Optional Termination Right, Percent Of Reinsurance Coverage Threshold

+ References

No definition available.

+ Details

Name:	nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold
Namespace Prefix:	nmih_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

- 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

- 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

No definition available.

+ Details

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

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21409-158489

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99380617&loc=SL75241803-196195

+ Details

Name:	us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Stated amount that is the upper retention limit per entity policy.

+ References

No definition available.

+ Details

Name:	us-gaap_ReinsuranceRetentionPolicyAmountRetained
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of risk reinsured in excess of stated retention per company policy.

+ References

No definition available.

+ Details

Name:	us-gaap_ReinsuranceRetentionPolicyExcessRetentionAmountReinsured
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of risk reinsured on insurance products per entity policy.

+ References

No definition available.

+ Details

Name:	us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=98513485&loc=SL98516268-108586

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(1)(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(2))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

Name:	us-gaap_RestrictedCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_ILNTransactionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nmih_TwoThousandSeventeenILNNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=nmih_OaktownReLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nmih_TwoThousandEighteenILNNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_TwoThousandSeventeenILNTransactionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_TwoThousandEighteenILNTransactionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_QSRTransactionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_TwoThousandEighteenQSRTransactionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_EligiblePoliciesPoolFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_EligiblePoliciesPoolFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_TwoThousandSixteenQSRTransactionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_EligiblePoliciesPoolOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_EligiblePoliciesPoolTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=nmih_EligiblePoliciesPoolThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Reinsurance - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended			6 Months Ended
Reinsurance - Narrative (Details) - USD ($) $ in Thousands	Apr. 01, 2019	Jul. 31, 2018	May 31, 2017	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Restricted cash				$ 1,430			$ 1,430		$ 1,414
Reinsurance recoverable on unpaid claims				$ 3,775		$ 2,382	$ 3,775	$ 2,382	$ 3,001	$ 1,902
ILN Transactions
Ceded Credit Risk [Line Items]
Optional termination right, percent of reinsurance coverage threshold				10.00%			10.00%
Mortgage-linked Debt \| 2017 ILN Notes \| Oaktown Re Ltd
Ceded Credit Risk [Line Items]
Proceeds from issuance of notes			$ 211,300
Mortgage-linked Debt \| 2018 ILN Notes \| Oaktown Re Ltd
Ceded Credit Risk [Line Items]
Proceeds from issuance of notes		$ 264,500
Third-Party Reinsurers \| 2017 ILN Transaction
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage							$ 91,500
Aggregate excess of loss reinsurance retained by company			126,800				124,400
Risk premiums paid				$ 1,300		$ 1,600	2,600	$ 3,300
Third-Party Reinsurers \| 2017 ILN Transaction \| Maximum
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage			$ 211,300
Reinsurance coverage, term of underlying mortgage amortization (in years)			10 years
Anticipated payment related to annual operating expenses							300
Third-Party Reinsurers \| 2018 ILN Transaction
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage							258,200
Aggregate excess of loss reinsurance retained by company		125,300					125,200
Risk premiums paid				1,600			$ 3,300
Third-Party Reinsurers \| 2018 ILN Transaction \| Maximum
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage		$ 264,500
Reinsurance coverage, term of underlying mortgage amortization (in years)		10 years
Anticipated payment related to annual operating expenses					$ 250
Third-Party Reinsurers \| QSR Transactions
Ceded Credit Risk [Line Items]
Ceding commissions under 2016 QSR Transaction							20.00%
Third-Party Reinsurers \| 2018 QSR Transaction
Ceded Credit Risk [Line Items]
Threshold for loss ratio on loans to qualify for profit commission							61.00%
Reinsurance recoverable on unpaid claims				$ 1,100			$ 1,100
Third-Party Reinsurers \| Risk Written Policies in 2018
Ceded Credit Risk [Line Items]
Percent of premiums earned under 2018 QSR Transaction							25.00%
Third-Party Reinsurers \| Risk Written Policies in 2019
Ceded Credit Risk [Line Items]
Percent of premiums ceded under QSR Transaction							20.00%
Third-Party Reinsurers \| 2016 QSR Transaction
Ceded Credit Risk [Line Items]
Percent of premiums ceded under QSR Transaction				20.50%	25.00%
Previously ceded primary RIF recaptured	$ 500,000
Threshold for loss ratio on loans to qualify for profit commission							60.00%
Reinsurance recoverable on unpaid claims				$ 2,700			$ 2,700
Third-Party Reinsurers \| Existing Risk Written Policies
Ceded Credit Risk [Line Items]
Percent of premiums ceded under QSR Transaction							25.00%
Third-Party Reinsurers \| Fannie Mae
Ceded Credit Risk [Line Items]
Percent of premiums ceded under QSR Transaction							100.00%
Third-Party Reinsurers \| Risk Written Policies from September 1, 2016 through December 31, 2017
Ceded Credit Risk [Line Items]
Percent of premiums ceded under QSR Transaction							25.00%