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Consolidated Statements Of Stockholders' Equity (Parenthetical) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2012
Quarter 1 Dividend Payment [Member]    
Regular dividends per share paid $ 1.00npk_RegularDividendsPerSharePaid
/ us-gaap_DividendsAxis
= npk_QuarterOneDividendPaymentMember		 $ 1.00npk_RegularDividendsPerSharePaid
/ us-gaap_DividendsAxis
= npk_QuarterOneDividendPaymentMember
Extra dividends per share paid $ 4.05npk_ExtraDividendsPerSharePaid
/ us-gaap_DividendsAxis
= npk_QuarterOneDividendPaymentMember		 $ 5.00npk_ExtraDividendsPerSharePaid
/ us-gaap_DividendsAxis
= npk_QuarterOneDividendPaymentMember
Quarter 4 Dividend Payment [Member]    
Regular dividends per share paid   $ 1.00npk_RegularDividendsPerSharePaid
/ us-gaap_DividendsAxis
= npk_QuarterFourDividendPaymentMember
Extra dividends per share paid   $ 5.50npk_ExtraDividendsPerSharePaid
/ us-gaap_DividendsAxis
= npk_QuarterFourDividendPaymentMember