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Consolidated Statements of Stockholders' Equity (Unaudited) (Parentheticals) - $ / shares	9 Months Ended
	Sep. 28, 2025	Sep. 29, 2024
Regular Dividends Per Share Paid (in dollars per share)	$ 1	$ 1
Extra Dividends Per Share Paid (in dollars per share)		$ 3.5

X

- Definition

Represents the extra dividends per share paid.

+ References

No definition available.

+ Details

Name:	npk_ExtraDividendsPerSharePaid
Namespace Prefix:	npk_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the regular dividends per share paid.

+ References

No definition available.

+ Details

Name:	npk_RegularDividendsPerSharePaid
Namespace Prefix:	npk_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration