Organization, Basis of Presentation and Summary of Accounting Principles (Policies)	6 Months Ended
Jun. 30, 2025
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, 2024, included in our 2024 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. In the opinion of management, these unaudited condensed consolidated financial statements include all adjustments (including normal recurring adjustments) that are necessary to present a fair statement of financial position, results of operations and cash flows for the periods presented. 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, 2025.
Recent Accounting Pronouncements - Adopted and Not Yet Adopted	Recent Accounting Pronouncements – Adopted In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740). The update enhances the disclosure requirements related to tax rate reconciliations and income taxes paid, and took effect for all public business entities for annual reporting periods beginning after December 15, 2024. We adopted this ASU on January 1, 2025 and will include enhanced disclosures in our fiscal year-end 2025 annual consolidated financial statements, as applicable. Recent Accounting Pronouncements – Not Yet Adopted In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expense (Topic 220). The update expands disclosure requirements related to certain income statement expenses, including a requirement to provide a tabular disaggregation of certain operating expenses by category. The standard will take effect for all public business entities for annual reporting periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements.
Fair Value of Financial Instruments	The following describes the valuation techniques used by us to determine the fair value of our financial instruments: We established a fair value hierarchy by prioritizing the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy under this standard are described below: Level 1 – Fair value measurements based on quoted prices in active markets that we have the ability to access for identical assets or liabilities. Market price data generally is obtained from exchange or dealer markets. We do not adjust the quoted price for such instruments. Level 2 – Fair value measurements based on inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs other than quoted prices that are observable for the asset or liability, such as interest rates and yield curves that are observable at commonly quoted intervals. Level 3 – Fair value measurements based on valuation techniques that use significant inputs that are unobservable. Both observable and unobservable inputs may be used to determine the fair values of positions classified in Level 3. The circumstances for using these measurements include those in which there is little, if any, market activity for the asset or liability. Therefore, we must make certain assumptions, which require significant management judgment or estimation about the inputs a hypothetical market participant would use to value that asset or liability. In certain cases, the inputs used to measure fair value may fall into different levels of the fair value hierarchy. In such cases, the level in the fair value hierarchy is determined based on the lowest level input that is significant to the fair value measurement in its entirety. Assets Classified as Level 1 and Level 2 To determine the fair value of securities available-for-sale in Level 1 and Level 2 of the fair value hierarchy, independent pricing sources have been utilized. One price is provided per security based on observable market data. To ensure securities are appropriately classified in the fair value hierarchy, we review the pricing techniques and methodologies of the independent pricing sources and believe that their policies adequately consider market activity, either based on specific transactions for the issue valued or based on modeling of securities with similar credit quality, duration, yield and structure that were recently traded. A variety of inputs are utilized by the independent pricing sources including benchmark yields, reported trades, non-binding broker/dealer quotes, issuer spreads, two sided markets, benchmark securities, bids, offers and reference data including data published in market research publications. Inputs may be weighted differently for any security, and not all inputs are used for each security evaluation. Market indicators, industry and economic events are also considered. This information is evaluated using a multidimensional pricing model. Quality controls are performed by the independent pricing sources throughout this process, which include reviewing tolerance reports, trading information and data changes, and directional moves compared to market moves. This model combines all inputs to arrive at a value assigned to each security. We have not made any adjustments to the prices obtained from the independent pricing sources.
Earnings per Share (EPS)	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.
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 associated receivable is written off against earned premium and the related insurance policy is canceled. Premiums receivable may be written off prior to 120 days in the ordinary course of business for non-credit events including, but not limited to, the modification or refinancing of an underlying insured loan. We established a $1.6 million and $2.2 million reserve for premium write-offs at June 30, 2025 and December 31, 2024, respectively. We separately 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 (a necessary precondition for access to mortgage credit for covered borrowers) and the short duration of the related receivables, we do not typically experience credit losses against our premium receivables and the allowance for credit loss established on premium receivable was deemed immaterial at June 30, 2025 and December 31, 2024.

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

6 Months Ended

Jun. 30, 2025

Organization, Consolidation and Presentation of Financial Statements [Abstract]

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, 2024, included in our 2024 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. In the opinion of management, these unaudited condensed consolidated financial statements include all adjustments (including normal recurring adjustments) that are necessary to present a fair statement of financial position, results of operations and cash flows for the periods presented. 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, 2025.

Recent Accounting Pronouncements - Adopted and Not Yet Adopted

Recent Accounting Pronouncements – Adopted

In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740). The update enhances the disclosure requirements related to tax rate reconciliations and income taxes paid, and took effect for all public business entities for annual reporting periods beginning after December 15, 2024. We adopted this ASU on January 1, 2025 and will include enhanced disclosures in our fiscal year-end 2025 annual consolidated financial statements, as applicable.

Recent Accounting Pronouncements – Not Yet Adopted

In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expense (Topic 220). The update expands disclosure requirements related to certain income statement expenses, including a requirement to provide a tabular disaggregation of certain operating expenses by category. The standard will take effect for all public business entities for annual reporting periods beginning after December 15, 2026, and interim reporting periods beginning after December 15, 2027. Early adoption is permitted. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements.

Fair Value of Financial Instruments

The following describes the valuation techniques used by us to determine the fair value of our financial instruments:

We established a fair value hierarchy by prioritizing the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy under this standard are described below:

Level 1 – Fair value measurements based on quoted prices in active markets that we have the ability to access for identical assets or liabilities. Market price data generally is obtained from exchange or dealer markets. We do not adjust the quoted price for such instruments.

Level 2 – Fair value measurements based on inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs other than quoted prices that are observable for the asset or liability, such as interest rates and yield curves that are observable at commonly quoted intervals.

Level 3 – Fair value measurements based on valuation techniques that use significant inputs that are unobservable. Both observable and unobservable inputs may be used to determine the fair values of positions classified in Level 3. The circumstances for using these measurements include those in which there is little, if any, market activity for the asset or liability. Therefore, we must make certain assumptions, which require significant management judgment or estimation about the inputs a hypothetical market participant would use to value that asset or liability.

In certain cases, the inputs used to measure fair value may fall into different levels of the fair value hierarchy. In such cases, the level in the fair value hierarchy is determined based on the lowest level input that is significant to the fair value measurement in its entirety.

Assets Classified as Level 1 and Level 2

To determine the fair value of securities available-for-sale in Level 1 and Level 2 of the fair value hierarchy, independent pricing sources have been utilized. One price is provided per security based on observable market data. To ensure securities are appropriately classified in the fair value hierarchy, we review the pricing techniques and methodologies of the independent pricing sources and believe that their policies adequately consider market activity, either based on specific transactions for the issue valued or based on modeling of securities with similar credit quality, duration, yield and structure that were recently traded. A variety of inputs are utilized by the independent pricing sources including benchmark yields, reported trades, non-binding broker/dealer quotes, issuer spreads, two sided markets, benchmark securities, bids, offers and reference data including data published in market research publications. Inputs may be weighted differently for any security, and not all inputs are used for each security evaluation. Market indicators, industry and economic events are also considered. This information is evaluated using a multidimensional pricing model. Quality controls are performed by the independent pricing sources throughout this process, which include reviewing tolerance reports, trading information and data changes, and directional moves compared to market moves. This model combines all inputs to arrive at a value assigned to each security. We have not made any adjustments to the prices obtained from the independent pricing sources.

Earnings per Share (EPS)

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.

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 associated receivable is written off against earned premium and the related insurance policy is canceled. Premiums receivable may be written off prior to 120 days in the ordinary course of business for non-credit events including, but not limited to, the modification or refinancing of an underlying insured loan. We established a $1.6 million and $2.2 million reserve for premium write-offs at June 30, 2025 and December 31, 2024, respectively.

We separately 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 (a necessary precondition for access to mortgage credit for covered borrowers) and the short duration of the related receivables, we do not typically experience credit losses against our premium receivables and the allowance for credit loss established on premium receivable was deemed immaterial at June 30, 2025 and December 31, 2024.