Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Abstract]
Doubtful accounts	$ 3,365	$ 3,202
Insurance (primarily product liability)	1,994npk_DeferredTaxAssetsInsurance	2,224npk_DeferredTaxAssetsInsurance
Vacation	954	907
Inventory	778us-gaap_DeferredTaxAssetsInventory	557us-gaap_DeferredTaxAssetsInventory
Goodwill and other intangibles	4,239	296
Other	905us-gaap_DeferredTaxAssetsOther	897us-gaap_DeferredTaxAssetsOther
Total deferred tax assets	12,235us-gaap_DeferredTaxAssetsNet	8,083us-gaap_DeferredTaxAssetsNet
Depreciation	8,529npk_DeferredTaxLiabilitiesDepreciation	6,755npk_DeferredTaxLiabilitiesDepreciation
State tax refunds	1,371npk_StateTaxRefunds
Other		4us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities	9,900us-gaap_DeferredTaxLiabilities	6,759us-gaap_DeferredTaxLiabilities
Net deferred tax assets (liabilities)	$ 2,335us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 1,324us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred tax assets, insurance

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Deferred tax liabilities, depreciation

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State tax refunds

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from intangible assets including goodwill.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from inventory.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from compensated absences (includes, but not limited to, sick and personal days).

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the allowance for doubtful accounts.

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Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

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Amount of deferred tax liability attributable to taxable temporary differences not separately disclosed.

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