Organization, Basis of Presentation and Summary of Accounting Principles (Policies)	3 Months Ended
Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements, which include the results of NMIH and its wholly-owned subsidiaries, have been prepared in accordance with the instructions to Form 10-Q as prescribed by the SEC for interim reporting and include other information and disclosures required by accounting principles generally accepted in the U.S. (GAAP). Our accounts are maintained in U.S. dollars. These statements should be read in conjunction with our consolidated financial statements and notes thereto for the year ended December 31, 2019, included in our 2019 10-K. All intercompany transactions have been eliminated. The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect reported amounts of assets and liabilities, as well as disclosure of contingent assets and liabilities as of the balance sheet date. Estimates also affect the reported amounts of income and expenses for the reporting period. Actual results could differ from those estimates. Certain reclassifications to previously reported financial information have been made to conform to current period presentation. The results of operations for the interim period may not be indicative of the results that may be expected for the full year ending December 31, 2020.
Investments	Investments We evaluate our investments each quarter to determine whether declines in fair value are below amortized cost. For any impaired debt security we evaluate if we either intend to sell the security or it is more likely than not that we will be required to sell the security before recovery. If our intent is to sell a security and its fair value is below amortized cost, the securities amortized cost basis is written down to fair value and a loss is recognized in earnings. For available for sale debt securities that do not meet the aforementioned criteria, we evaluate whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, we consider several factors including, but not limited to: •the severity of the decline in fair value; •the financial condition of the issuer; •the failure of the issuer to make scheduled interest or principal payments; •recent credit downgrades of the applicable security or the issuer; and •adverse conditions specifically related to the security, an industry, or a geographic area. If this assessment indicates that a credit loss exists, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of the cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses is recorded for the credit loss, limited by the amount that the fair value is less than the amortized cost basis. Any impairment that has not been recorded through the allowance for credit losses is recognized in other comprehensive income. Changes in allowance for credit losses are recorded as provision for (or reversal of) credit loss expense. Losses are charged against the allowance for credit losses when management believes the uncollectibility of a security is met. We have elected to present accrued interest receivable separately from available for sale securities on the consolidated balance sheet in "Accrued Investment Income." Accrued interest receivable was $7.2 million as of March 31, 2020 relating almost entirely to available for sale debt securities. We have also elected not to measure an allowance for credit losses for accrued interest receivable on available for sale securities. For all classes of available for sale securities, the accrual of interest is discontinued when the contractual payment of principal or interest has become 90 days past due and write-off of accrued interest receivable is recognized by reversing interest income. We did not write off any accrued interest receivable during the three months ended March 31, 2020.
Premiums Receivable	Premiums Receivable Premiums receivable consists of premiums due on our mortgage insurance policies. If a mortgage insurance premium is unpaid for more than 120 days, the receivable is written off against earned premium and the related insurance policy is canceled. We recognize an allowance for credit losses for premiums receivable based on credit losses expected to arise over the life of the receivable. Due to the nature of our insurance policies and the short-term duration of the related receivables an allowance for credit loss was not required at March 31, 2020.
Reinsurance	Reinsurance NMIC has entered into quota share reinsurance treaties effective September 1, 2016 (the 2016 QSR Transaction) and January 1, 2018 (the 2018 QSR Transaction), which we refer to collectively as the QSR Transactions. Under the QSR Transactions, we cede a portion of claims and claim expense reserves to our reinsurers, and account for such ceded reserves as reinsurance recoverables in "Other Assets" on the consolidated balance sheets and as reductions to claims expenses on the consolidated statements of operations. The company actively manages and monitors its credit risk associated with its reinsurance recoverables and recognizes an allowance for credit losses based on losses expected to arise over the life of the asset. As it relates to its QSR transactions, each of the third-party reinsurance providers has an insurer financial strength rating of A- or better by Standard & Poor's Rating Service (S&P), A.M. Best Company, Inc. (A.M.Best) or both. In accordance with the terms of the 2016 QSR Transaction, rather than making a cash payment or transferring investments for ceded premiums written, NMIC established a funds withheld liability, which also includes amounts due to NMIC for ceding and profit commissions. Any loss recoveries and any potential profit commission to NMIC will be realized from this account until exhausted. In accordance with the terms of the 2018 QSR Transaction, cash payments for ceded premiums earned are settled on a quarterly basis, offset by amounts due to NMIC for ceding and profit commissions. Any loss recoveries and any potential profit commission to NMIC are also settled quarterly. NMIC's reinsurance recoverable balances are further supported by trust accounts established and maintained by each reinsurer in accordance with the private mortgage insurer eligibility requirements (PMIERs) funding requirements for risk ceded to non-affiliates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements - Adopted In June 2016, the Financial Accounting Standard Board (FASB) issued Accounting Standards Update (ASU) 2016-13, Financial Instruments-Credit Losses (Topic 326) and subsequently issued amendments to the initial guidance: ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments-Credit Losses (Topic 815), Derivatives and Hedging, and Topic 825, Financial Instruments, ASU 2019-05, Financial Instruments-Credit Losses: Targeted Transition Relief, and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments-Credit Losses. These updates will require companies to measure and establish reserves for lifetime expected credit losses on many financial assets held at a given reporting date. Under the guidance, the methodology for measuring lifetime credit losses shifts from an incurred loss model, whereby losses are only recognized once probable and estimable, to a current expected credit loss (CECL) model, whereby losses are recognized upfront based on a future economic forecast. Credit losses relating to available-for-sale fixed maturity securities are recorded through an allowance for credit losses, rather than a write-down of the asset as was required, with the amount of the allowance limited to the amount by which fair value is less than amortized cost. The length of time an available-for sale fixed maturity security has been held in an unrealized loss position will no longer impact its credit loss determination. We adopted these updates on January 1, 2020. Adoption of the updated standards did not have a material impact on our consolidated financial statements, and had no impact on our accounting for insurance claims and claim expenses as these items are not in scope of the guidance. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820). This update modifies the fair value measurement disclosure requirements of ASC 820. We adopted this ASU on January 1, 2020 and determined it did not have a material impact on our consolidated financial statements. In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40). This update applies to cloud computing arrangements structured as service contracts, and provides companies with guidance on the criteria for capitalizing implementation, set-up and other up-front costs incurred in association with these arrangements. We adopted this ASU on January 1, 2020 and applied it on a prospective basis for eligible costs incurred after the effective date. The adoption of this ASU did not have a material impact on our consolidated financial statements. Recent Accounting Pronouncements - Not Yet Adopted In August 2018, the FASB issued ASU 2018-12, Targeted Improvements to the Accounting for Long-Duration Contracts. This update provides guidance to the existing recognition, measurement, presentation and disclosure requirements for long-duration contracts issued by an insurance entity. The FASB subsequently issued ASU 2019-09 in November 2019, which amended the effective date for this standard. The standard will now take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2021. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements. In November 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740). This update eliminates certain exceptions for recognizing deferred taxes for investments, performing intra-period allocations and calculating income taxes in interim periods. The ASU also includes guidance to reduce complexity in certain income tax areas, including recognizing deferred taxes for tax goodwill and allocating taxes to members of a consolidated group. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements. In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848). This update provides optional guidance to ease the potential burden in accounting for reference rate reform on financial reporting. Reference rate reform refers to the global transition away from referencing the London Interbank Offered Rate (LIBOR) in financial contracts, which is expected to be discontinued in 2021. The ASU includes optional expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform if certain criteria are met. The standard is effective immediately through December 31, 2022 for all entities that have contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. We are currently evaluating the impact the adoption of this ASU would have, if any, to our contract modifications that are affected by the discontinuation of LIBOR.
Earnings Per Share	Basic EPS is based on the weighted average number of shares of common stock outstanding. Diluted EPS is based on the weighted average number of shares of common stock outstanding and common stock equivalents that would be issuable upon the vesting of service based and performance and service based restricted stock units (RSUs), and the exercise of vested and unvested stock options and outstanding warrants. The number of shares issuable for RSUs subject to performance and service based vesting requirements are only included in diluted shares if the relevant performance measurement period has commenced and results during such period meet the necessary performance criteria.

Organization, Basis of Presentation and Summary of Accounting Principles (Policies)

3 Months Ended

Mar. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of Presentation The accompanying unaudited condensed consolidated financial statements, which include the results of NMIH and its wholly-owned subsidiaries, have been prepared in accordance with the instructions to Form 10-Q as prescribed by the SEC for interim reporting and include other information and disclosures required by accounting principles generally accepted in the U.S. (GAAP). Our accounts are maintained in U.S. dollars. These statements should be read in conjunction with our consolidated financial statements and notes thereto for the year ended December 31, 2019, included in our 2019 10-K. All intercompany transactions have been eliminated. The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect reported amounts of assets and liabilities, as well as disclosure of contingent assets and liabilities as of the balance sheet date. Estimates also affect the reported amounts of income and expenses for the reporting period. Actual results could differ from those estimates. Certain reclassifications to previously reported financial information have been made to conform to current period presentation. The results of operations for the interim period may not be indicative of the results that may be expected for the full year ending December 31, 2020.

Investments

We evaluate our investments each quarter to determine whether declines in fair value are below amortized cost. For any impaired debt security we evaluate if we either intend to sell the security or it is more likely than not that we will be required to sell the security before recovery. If our intent is to sell a security and its fair value is below amortized cost, the securities amortized cost basis is written down to fair value and a loss is recognized in earnings.

For available for sale debt securities that do not meet the aforementioned criteria, we evaluate whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, we consider several factors including, but not limited to:

•the severity of the decline in fair value;

•the financial condition of the issuer;

•the failure of the issuer to make scheduled interest or principal payments;

•recent credit downgrades of the applicable security or the issuer; and

•adverse conditions specifically related to the security, an industry, or a geographic area.

If this assessment indicates that a credit loss exists, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of the cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses is recorded for the credit loss, limited by the amount that the fair value is less than the amortized cost basis. Any impairment that has not been recorded through the allowance for credit losses is recognized in other comprehensive income.

Changes in allowance for credit losses are recorded as provision for (or reversal of) credit loss expense. Losses are charged against the allowance for credit losses when management believes the uncollectibility of a security is met.

We have elected to present accrued interest receivable separately from available for sale securities on the consolidated balance sheet in "Accrued Investment Income." Accrued interest receivable was $7.2 million as of March 31, 2020 relating almost entirely to available for sale debt securities. We have also elected not to measure an allowance for credit losses for accrued interest receivable on available for sale securities. For all classes of available for sale securities, the accrual of interest is discontinued when the contractual payment of principal or interest has become 90 days past due and write-off of accrued interest receivable is recognized by reversing interest income. We did not write off any accrued interest receivable during the three months ended March 31, 2020.

Premiums Receivable

Premiums Receivable Premiums receivable consists of premiums due on our mortgage insurance policies. If a mortgage insurance premium is unpaid for more than 120 days, the receivable is written off against earned premium and the related insurance policy is canceled. We recognize an allowance for credit losses for premiums receivable based on credit losses expected to arise over the life of the receivable. Due to the nature of our insurance policies and the short-term duration of the related receivables an allowance for credit loss was not required at March 31, 2020.

Reinsurance

NMIC has entered into quota share reinsurance treaties effective September 1, 2016 (the 2016 QSR Transaction) and January 1, 2018 (the 2018 QSR Transaction), which we refer to collectively as the QSR Transactions. Under the QSR Transactions, we cede a portion of claims and claim expense reserves to our reinsurers, and account for such ceded reserves as reinsurance recoverables in "Other Assets" on the consolidated balance sheets and as reductions to claims expenses on the consolidated statements of operations. The company actively manages and monitors its credit risk associated with its reinsurance recoverables and recognizes an allowance for credit losses based on losses expected to arise over the life of the asset.

As it relates to its QSR transactions, each of the third-party reinsurance providers has an insurer financial strength rating of A- or better by Standard & Poor's Rating Service (S&P), A.M. Best Company, Inc. (A.M.Best) or both. In accordance with the terms of the 2016 QSR Transaction, rather than making a cash payment or transferring investments for ceded premiums written, NMIC established a funds withheld liability, which also includes amounts due to NMIC for ceding and profit commissions. Any loss recoveries and any potential profit commission to NMIC will be realized from this account until exhausted. In accordance with the terms of the 2018 QSR Transaction, cash payments for ceded premiums earned are settled on a quarterly basis, offset by amounts due to NMIC for ceding and profit commissions. Any loss recoveries and any potential profit commission to NMIC are also settled quarterly. NMIC's reinsurance recoverable balances are further supported by trust accounts established and maintained by each reinsurer in accordance with the private mortgage insurer eligibility requirements (PMIERs) funding requirements for risk ceded to non-affiliates.

Recent Accounting Pronouncements

Recent Accounting Pronouncements - Adopted

In June 2016, the Financial Accounting Standard Board (FASB) issued Accounting Standards Update (ASU) 2016-13, Financial Instruments-Credit Losses (Topic 326) and subsequently issued amendments to the initial guidance: ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments-Credit Losses (Topic 815), Derivatives and Hedging, and Topic 825, Financial Instruments, ASU 2019-05, Financial Instruments-Credit Losses: Targeted Transition Relief, and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments-Credit Losses. These updates will require companies to measure and establish reserves for lifetime expected credit losses on many financial assets held at a given reporting date. Under the guidance, the methodology for measuring lifetime credit losses shifts from an incurred loss model, whereby losses are only recognized once probable and estimable, to a current expected credit loss (CECL) model, whereby losses are recognized upfront based on a future economic forecast. Credit losses relating to available-for-sale fixed maturity securities are recorded through an allowance for credit losses, rather than a write-down of the asset as was required, with the amount of the allowance limited to the amount by which fair value is less than amortized cost. The length of time an available-for sale fixed maturity security has been held in an unrealized loss position will no longer impact its credit loss determination. We adopted these updates on January 1, 2020. Adoption of the updated standards did not have a material impact on our consolidated financial statements, and had no impact on our accounting for insurance claims and claim expenses as these items are not in scope of the guidance.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820). This update modifies the fair value measurement disclosure requirements of ASC 820. We adopted this ASU on January 1, 2020 and determined it did not have a material impact on our consolidated financial statements.

In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40). This update applies to cloud computing arrangements structured as service contracts, and provides companies with

guidance on the criteria for capitalizing implementation, set-up and other up-front costs incurred in association with these arrangements. We adopted this ASU on January 1, 2020 and applied it on a prospective basis for eligible costs incurred after the effective date. The adoption of this ASU did not have a material impact on our consolidated financial statements.

Recent Accounting Pronouncements - Not Yet Adopted

In August 2018, the FASB issued ASU 2018-12, Targeted Improvements to the Accounting for Long-Duration Contracts. This update provides guidance to the existing recognition, measurement, presentation and disclosure requirements for long-duration contracts issued by an insurance entity. The FASB subsequently issued ASU 2019-09 in November 2019, which amended the effective date for this standard. The standard will now take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2021. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements.

In November 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740). This update eliminates certain exceptions for recognizing deferred taxes for investments, performing intra-period allocations and calculating income taxes in interim periods. The ASU also includes guidance to reduce complexity in certain income tax areas, including recognizing deferred taxes for tax goodwill and allocating taxes to members of a consolidated group. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements.

In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848). This update provides optional guidance to ease the potential burden in accounting for reference rate reform on financial reporting. Reference rate reform refers to the global transition away from referencing the London Interbank Offered Rate (LIBOR) in financial contracts, which is expected to be discontinued in 2021. The ASU includes optional expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform if certain criteria are met. The standard is effective immediately through December 31, 2022 for all entities that have contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. We are currently evaluating the impact the adoption of this ASU would have, if any, to our contract modifications that are affected by the discontinuation of LIBOR.

Earnings Per Share

Basic EPS is based on the weighted average number of shares of common stock outstanding. Diluted EPS is based on the weighted average number of shares of common stock outstanding and common stock equivalents that would be issuable upon the vesting of service based and performance and service based restricted stock units (RSUs), and the exercise of vested and unvested stock options and outstanding warrants. The number of shares issuable for RSUs subject to performance and service based vesting requirements are only included in diluted shares if the relevant performance measurement period has commenced and results during such period meet the necessary performance criteria.