Document And Entity Information - shares	9 Months Ended
	Sep. 27, 2020	Nov. 06, 2020
Document Information [Line Items]
Entity Central Index Key	0000080172
Entity Registrant Name	NATIONAL PRESTO INDUSTRIES INC
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2020
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 27, 2020
Document Transition Report	false
Entity File Number	1-2451
Entity Incorporation, State or Country Code	WI
Entity Tax Identification Number	39-0494170
Entity Address, Address Line One	3925 North Hastings Way
Entity Address, City or Town	Eau Claire
Entity Address, State or Province	WI
Entity Address, Postal Zip Code	54703-3703
City Area Code	715
Local Phone Number	839-2121
Title of 12(b) Security	Common Stock, $1 par value
Trading Symbol	NPK
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		7,023,288