Document And Entity Information Statistical Measurement [Axis] Note To Financial Statement Details Textual Note D - Business Segments Note F - Cash, Cash Equivalents And Marketable Securities Provision for income taxes Fair Value Measurement Inputs and Valuation Techniques [Table] ecd_NonRule10b51ArrTrmntdFlag Non-Rule 10b5-1 Arrangement Terminated [Flag] Note H - Leases ecd_NonRule10b51ArrAdoptedFlag Non-Rule 10b5-1 Arrangement Adopted [Flag] Note D - Business Segments - Segment Information (Details) Trading Arrangements, by Individual [Table] Provision for depreciation Note F - Cash, Cash Equivalents And Marketable Securities - Summary of Marketable Securities (Details) Material Terms of Trading Arrangement [Text Block] Note H - Leases - Lease Cost (Details) Note H - Leases - Lease Maturities (Details) Note H - Leases - Operating Lease Income (Details) Financial Instrument [Axis] Notes To Financial Statements Notes To Financial Statements [Abstract] Minimum [Member] Gross Unrealized Losses Gross Unrealized Gains Statistical Measurement [Domain] Maximum [Member] Cash dividends declared and paid per common share (in dollars per share) Represents cash dividends declared and paid. Non-cash retirement plan expense Represents the amount of noncash retirement plan expense. us-gaap_IncreaseDecreaseInAccountsReceivable Accounts receivable, net Accounts payable and accrued liabilities Certificates of Deposit [Member] Cash flows from operating activities: Cash flows from investing activities: us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash provided by operating activities Cash flows from financing activities: us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash provided by (used in) investing activities us-gaap_NetCashProvidedByUsedInFinancingActivities Net cash used in financing activities Finished goods Thereafter, maturities npk_LesseeOperatingLeaseLiabilityToBePaidAfterYearFour Amount of lessee's undiscounted obligation for lease payment for operating lease due after fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). npk_LessorOperatingLeasePaymentToBeReceivedAfterYearFour Thereafter, lease income Amount of lease payment to be received by lessor for operating lease after fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). Regular dividends per share (in dollars per share) Represents the regular dividends per share paid. npk_SupplyCommitmentCommitmentTerm Supply Commitment, Commitment Term (Year) The commitment term of supply commitment. npk_RevenueContractPeriod Revenue Contract Period (Month) Represents the revenue contract period. Housewares/Small Appliances [Member] Information pertaining to the Housewares / Small Appliances segment. Operating Segments [Member] Accounting Policies [Abstract] Defense [Member] Information pertaining to the Defense segment. npk_GovernmentContractNumberOfContractors Government Contract, Number of Contractors The number of contractors of government contract. Segments [Domain] Safety [Member] Information pertaining to the Safety segment. us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable Federal and state income taxes Accounting Standards Update and Change in Accounting Principle [Text Block] Financial Instruments [Domain] us-gaap_MaterialsSuppliesAndOther Materials, Supplies, and Other Segments [Axis] Short-term and variable lease cost Amount of short-term and variable lease cost. Cash, Cash Equivalent, and Marketable Security [Text Block] us-gaap_IncreaseDecreaseInInventories Inventories Income Statement [Abstract] npk_ProceedsFromSaleOfPropertyPlantAndEquipmentGross Proceeds from Sale of Property, Plant, and Equipment, Gross The gross amount of cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. us-gaap_NumberOfOperatingSegments Number of Operating Segments us-gaap_NumberOfReportableSegments Number of Reportable Segments Segment Reporting [Text Block] Treasury stock, at cost Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] Treasury Stock, Common [Member] Segment Reporting [Table Text Block] Net earnings Net earnings Adjustments to reconcile net earnings to net cash provided by operating activities: us-gaap_Liabilities Total liabilities Intangibles amortization us-gaap_LiabilitiesAndStockholdersEquity Liabilities and Equity Consolidation Items [Axis] Cash and Cash Equivalent [Table] Segment Reporting [Table] us-gaap_OtherNoncashIncomeExpense Other Selling and general expenses LIABILITIES AND STOCKHOLDERS' EQUITY Consolidation Items [Domain] Lessee, Lease, Description [Table] Changes in operating accounts: Accounting Standards Update and Change in Accounting Principle [Table] us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect Net increase (decrease) in cash and cash equivalents Comprehensive income: Statement [Table] Line of Credit us-gaap_OperatingLeasePayments Operating Lease, Payments Credit Facility [Axis] Servicing Asset, Impairment, Valuation Allowance, Activity [Table] Credit Facility [Domain] Revolving Credit Facility [Member] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Domestic Line of Credit [Member] us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 Debt Instrument, Basis Spread on Variable Rate us-gaap_RepaymentsOfLinesOfCredit Payments on line of credit Statement of Financial Position [Abstract] us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity Line of Credit Facility, Maximum Borrowing Capacity Deposit Liability [Table] Income tax receivable Proceeds from note receivable us-gaap_DividendsCommonStock Dividends paid March 17, $1.00 per share regular us-gaap_LineOfCredit Long-Term Line of Credit Line of Credit Facility [Table] us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue Total lease payments us-gaap_IncreaseDecreaseInOtherOperatingAssets Other assets and current assets us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent Operating Lease, Weighted Average Discount Rate, Percent Document Type Document Information [Line Items] Document Quarterly Report 2027, maturities us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1 Operating Lease, Weighted Average Remaining Lease Term (Year) Document Information [Table] us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1 Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month) 2026 (remaining nine months), maturities us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear Entity Interactive Data Current Lessee, Operating Lease, Liability, to be Paid, Maturity [Table Text Block] 2029, maturities us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree Commitments and Contingencies Disclosure [Text Block] Document Transition Report Marketable securities - maturities and sales Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale Warehousing Facility, Canton, Mississippi [Member] Represents sold warehousing facility in Canton, Mississippi. 2028, maturities us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo Statement [Line Items] 2030, maturities us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour Common stock, issued (in shares) Net Earnings per share: us-gaap_CommonStockSharesOutstanding Balance (in shares) Balance (in shares) us-gaap_LessorOperatingLeasePaymentsToBeReceivedFourYears 2030, lease income us-gaap_LessorOperatingLeasePaymentsToBeReceived Total lease payments Common stock, authorized (in shares) Operating Lease, Lease Income [Table Text Block] us-gaap_LessorOperatingLeasePaymentsToBeReceivedNextTwelveMonths 2027, lease income Basic and diluted (in shares) us-gaap_OperatingLeaseLeaseIncomeLeasePayments Operating Lease, Lease Income, Lease Payments Antidilutive Security, Excluded EPS Calculation [Table] us-gaap_LessorOperatingLeasePaymentsToBeReceivedRemainderOfFiscalYear 2026 (remaining nine months), lease income Basic and diluted (in dollars per share) us-gaap_LessorOperatingLeasePaymentsToBeReceivedThreeYears 2029, lease income us-gaap_LessorOperatingLeasePaymentsToBeReceivedTwoYears 2028, lease income Common stock, $1 par value: Authorized: 12,000,000 shares: Issued: 7,440,518 shares us-gaap_PaymentsForDeposits Payments for Deposits Accounts payable INTANGIBLE ASSETS, net us-gaap_PolicyTextBlockAbstract Accounting Policies us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements us-gaap_TableTextBlock Notes Tables Proceeds from line of credit Other Document Fiscal Period Focus Document Fiscal Year Focus us-gaap_ComprehensiveIncomeNetOfTax Comprehensive income Proceeds from sale of treasury stock Amendment Flag Common Stock [Member] Subsequent Events [Text Block] GOODWILL Accrued liabilities Document Period End Date Legal Entity [Axis] Operating lease cost us-gaap_LeaseCost Total lease cost Less: future interest expense Entity File Number us-gaap_RevenueRemainingPerformanceObligation Revenue, Remaining Performance Obligation, Amount LEASE LIABILITIES - NON-CURRENT Entity Tax Identification Number Lease liabilities us-gaap_OperatingLeaseLiability Lease liabilities Disaggregation of Revenue [Table] Entity [Domain] Entity Central Index Key Entity Registrant Name Entity Current Reporting Status us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss Debt Securities, Available-for-Sale, Realized Gain (Loss) Entity Incorporation, State or Country Code Current Fiscal Year End Date Asset, Impairment [Text Block] Entity Filer Category Entity Shell Company Entity Emerging Growth Company Entity Small Business Amortized Cost Operating profit (loss) Operating profit Operating profit (loss) Proceeds from sale of property, plant and equipment us-gaap_ContractWithCustomerLiabilityRevenueRecognized Contract with Customer, Liability, Revenue Recognized Net sales External net sales Other comprehensive income, net of tax: Loss Contingencies [Table] Entity Address, State or Province Statement of Stockholders' Equity [Abstract] Entity Address, City or Town Retained earnings Entity Address, Address Line One us-gaap_StockholdersEquityBeforeTreasuryStock Stockholders' Equity before Treasury Stock Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Statement of Cash Flows [Abstract] City Area Code Local Phone Number Entity Address, Postal Zip Code Revenue from Contract with Customer [Text Block] us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value STOCKHOLDERS' EQUITY Title of 12(b) Security Equity Components [Axis] Security Exchange Name Trading Symbol Unrealized loss on available-for-sale securities, net of tax us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent Unrealized loss on available-for-sale securities Class of Stock [Axis] Equity Component [Domain] Fair Value Variable Rate [Domain] AOCI Attributable to Parent [Member] Entity Common Stock, Shares Outstanding Additional Paid-in Capital [Member] Retained Earnings [Member] Earnings Per Share [Text Block] Cost of sales Other current assets us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax Property, plant and equipment, net PROPERTY, PLANT AND EQUIPMENT Business Combination, Goodwill, Reportable Segment Assignment [Table] Subsequent Event [Table] FEDERAL AND STATE INCOME TAXES - NON-CURRENT DEFERRED INCOME TAXES Weighted average shares outstanding: Long-Lived Assets Held-for-Sale, Name [Domain] Common stock, par value (in dollars per share) Other income us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax us-gaap_Assets Assets Total assets Cash and cash equivalents Long-Lived Asset, Held-for-Sale, Type [Axis] Fair Value Disclosures [Text Block] Supplemental disclosures of cash flow information: Notes receivable, current Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Accounts receivable, net CURRENT LIABILITIES: us-gaap_LiabilitiesCurrent Total current liabilities us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration] us-gaap_LettersOfCreditOutstandingAmount Letters of Credit Outstanding, Amount us-gaap_ContractWithCustomerLiabilityCurrent Contract with Customer, Liability, Current Secured Overnight Financing Rate (SOFR) [Member] Depreciation and amortization Depreciation and amortization Marketable securities Lease, Cost [Table Text Block] Interest us-gaap_GainLossOnSaleOfPropertyPlantEquipment Gain on sale of property, plant and equipment Less allowance for depreciation Accumulated other comprehensive income Inventories: Disposal Groups, Including Discontinued Operations [Table] Inventory, net us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest Earnings before provision for income taxes Raw materials Paid-in capital Debt Disclosure [Text Block] Total stockholders' equity Balance Balance Work in process Other Assets Disclosure [Text Block] us-gaap_PaymentsOfDividendsCommonStock Dividends paid us-gaap_PaymentsToAcquirePropertyPlantAndEquipment Purchase of property, plant and equipment Capital expenditures Lessee, Operating Leases [Text Block] CURRENT ASSETS: RIGHT-OF-USE LEASE ASSETS us-gaap_AssetsCurrent Total current assets ecd_Rule10b51ArrTrmntdFlag Rule 10b5-1 Arrangement Terminated [Flag] ecd_Rule10b51ArrAdoptedFlag Rule 10b5-1 Arrangement Adopted [Flag] Gross profit (loss) Gross profit COMMITMENTS AND CONTINGENCIES