Consolidated Statements Of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Apr. 05, 2015	Mar. 30, 2014
Consolidated Statements Of Comprehensive Income [Abstract]
Net sales	$ 100,999us-gaap_SalesRevenueNet	$ 86,554us-gaap_SalesRevenueNet
Cost of sales	80,120us-gaap_CostOfGoodsAndServicesSold	70,834us-gaap_CostOfGoodsAndServicesSold
Gross profit	20,879us-gaap_GrossProfit	15,720us-gaap_GrossProfit
Selling and general expenses	6,161	6,295
Intangibles amortization	2,394us-gaap_AmortizationOfIntangibleAssets	2,443us-gaap_AmortizationOfIntangibleAssets
Operating profit	12,324us-gaap_OperatingIncomeLoss	6,982us-gaap_OperatingIncomeLoss
Other income	75us-gaap_OtherNonoperatingIncomeExpense	164us-gaap_OtherNonoperatingIncomeExpense
Earnings before provision for income taxes	12,399	7,146
Provision for income taxes	4,290us-gaap_IncomeTaxExpenseBenefit	2,456us-gaap_IncomeTaxExpenseBenefit
Net earnings	8,109us-gaap_NetIncomeLoss	4,690us-gaap_NetIncomeLoss
Weighted average common shares outstanding:
Basic and diluted	6,943	6,921
Net earnings per share:
Basic and diluted	$ 1.17us-gaap_EarningsPerShareBasicAndDiluted	$ 0.68us-gaap_EarningsPerShareBasicAndDiluted
Other comprehensive income, net of tax:
Unrealized gain on available-for-sale securities	7	4
Comprehensive income	$ 8,116us-gaap_ComprehensiveIncomeNetOfTax	$ 4,694us-gaap_ComprehensiveIncomeNetOfTax
Cash dividends declared and paid per common share	$ 4.05npk_CashDividendsDeclaredAndPaid	$ 5.05npk_CashDividendsDeclaredAndPaid

Cash dividends declared and paid.

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Aggregate revenue less cost of goods and services sold or operating expenses directly attributable to the revenue generation activity.

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The net result for the period of deducting operating expenses from operating revenues.

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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities, attributable to parent entity. Excludes amounts related to other than temporary impairment (OTTI) loss.

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The net amount of other income and expense amounts, the components of which are not separately disclosed on the income statement, resulting from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business) also known as other nonoperating income (expense) recognized for the period. Such amounts may include: (a) dividends, (b) interest on securities, (c) net gains or losses on securities, (d) unusual costs, (e) gains or losses on foreign exchange transactions, and (f) miscellaneous other income and expense items.

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Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

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The aggregate total costs related to selling a firm's product and services, as well as all other general and administrative expenses. Direct selling expenses (for example, credit, warranty, and advertising) are expenses that can be directly linked to the sale of specific products. Indirect selling expenses are expenses that cannot be directly linked to the sale of specific products, for example telephone expenses, Internet, and postal charges. General and administrative expenses include salaries of non-sales personnel, rent, utilities, communication, etc.

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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