Total amount of extra dividends declared per share during the period.

No definition available.

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Namespace Prefix:	npk_
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Period Type:	duration

- Definition

Total amount of regular dividends declared per share during the period.

+ References

No definition available.

+ Details

Name:	npk_RegularDividendsPerShareDeclared
Namespace Prefix:	npk_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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- Details

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Period Type:

Subsequent Event (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 21, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Dividends paid on common stock		$ 42,087	$ 41,989	$ 38,405
Subsequent Event [Member]
Subsequent Event [Line Items]
Regular dividends per share, declared	$ 1.00
Extra dividends per share, declared	$ 5.00