Subsequent Event (Narrative) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			1 Months Ended	3 Months Ended
	Dec. 31, 2014 employee	Sep. 28, 2014	Jun. 29, 2014	Mar. 30, 2014	Dec. 31, 2013	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 employee	Dec. 31, 2013	Dec. 31, 2012	Feb. 13, 2015	Mar. 13, 2015
Subsequent Event [Line Items]
Net sales	$ 142,034us-gaap_SalesRevenueNet	$ 95,463us-gaap_SalesRevenueNet	$ 88,312us-gaap_SalesRevenueNet	$ 86,554us-gaap_SalesRevenueNet	$ 134,990us-gaap_SalesRevenueNet	$ 100,612us-gaap_SalesRevenueNet	$ 101,396us-gaap_SalesRevenueNet	$ 83,190us-gaap_SalesRevenueNet	$ 412,363us-gaap_SalesRevenueNet	$ 420,188us-gaap_SalesRevenueNet	$ 472,490us-gaap_SalesRevenueNet
Dividends paid									34,954us-gaap_PaymentsOfDividendsCommonStock		86,106us-gaap_PaymentsOfDividendsCommonStock
Number of entity empolyees, union members	163npk_NumberOfEntityEmpolyeesUnionMembers								163npk_NumberOfEntityEmpolyeesUnionMembers
Percentage of entity employees, union members	16.00%								16.00%
Subsequent Event [Member]
Subsequent Event [Line Items]
Regular dividends per share paid												$ 1.00
Extra dividends per share paid												$ 3.05
Dividends paid													$ 28,114
Number of entity empolyees, union members	163								163
Percentage of entity employees, union members	16.00%								16.00%