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Inventories (Schedule Of Potential Impact Of LIFO Valuation to FIFO Valuation) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Inventories [Abstract]      
Cost of Sales $ 643us-gaap_InventoryLIFOReservePeriodCharge $ 1,941us-gaap_InventoryLIFOReservePeriodCharge $ (857)us-gaap_InventoryLIFOReservePeriodCharge
Net Earnings $ (422)us-gaap_InventoryLIFOReserveEffectOnIncomeNet $ (1,263)us-gaap_InventoryLIFOReserveEffectOnIncomeNet $ 546us-gaap_InventoryLIFOReserveEffectOnIncomeNet
Earnings Per Share $ (0.06)npk_InventoryLifoReserveEffectOnEarningsPerShare $ (0.18)npk_InventoryLifoReserveEffectOnEarningsPerShare $ 0.08npk_InventoryLifoReserveEffectOnEarningsPerShare