Note 6 - Investments in Joint Ventures - Condensed Balance Sheet of EXCOR and All Other Joint Ventures (Details) - USD ($)	6 Months Ended	12 Months Ended
	Feb. 28, 2019	Aug. 31, 2018
Current assets	$ 60,831,022	$ 58,086,747
Total assets	65,423,178	62,803,261
Current liabilities	15,529,788	15,991,886
Noncurrent liabilities	430,678	403,653
Joint ventures’ equity	49,462,712	46,407,722
Northern Technologies International Corporation’s share of joint ventures’ equity	24,418,491	22,950,995
Northern Technologies International Corporation’s share of joint ventures’ undistributed earnings	22,389,278	20,921,783
Northern Technologies International Corporation’s dividends received from joint ventures	2,027,243	6,377,054
EXCOR [Member]
Current assets	30,259,315	27,354,788
Total assets	32,737,188	30,033,750
Current liabilities	3,968,110	4,535,954
Noncurrent liabilities
Joint ventures’ equity	28,769,078	25,497,796
Northern Technologies International Corporation’s share of joint ventures’ equity	14,384,541	12,748,899
Northern Technologies International Corporation’s share of joint ventures’ undistributed earnings	14,353,636	12,717,994
Northern Technologies International Corporation’s dividends received from joint ventures	843,750	5,379,062
All Other [Member]
Current assets	30,571,707	30,731,959
Total assets	32,685,990	32,769,511
Current liabilities	11,561,678	11,455,932
Noncurrent liabilities	430,678	403,653
Joint ventures’ equity	20,693,634	20,909,926
Northern Technologies International Corporation’s share of joint ventures’ equity	10,033,950	10,195,263
Northern Technologies International Corporation’s share of joint ventures’ undistributed earnings	8,035,642	8,203,789
Northern Technologies International Corporation’s dividends received from joint ventures	$ 1,183,493	$ 997,992