Note 16 - Earnings Attributable to Tennant Company Per Share - Computations of Basic and Diluted Earnings (Loss) Attributable to Tennant Company Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net income attributable to Tennant Company	$ 21.5	$ 11.7	$ 57.0	$ 31.2
Basic - weighted average shares outstanding (in shares)	18,554,136	18,371,883	18,519,523	18,335,430
Share-based compensation plans (in shares)	317,081	276,445	350,375	288,537
Diluted - weighted average shares outstanding (in shares)	18,871,217	18,648,328	18,869,898	18,623,967
Basic earnings per share attributable to Tennant Company (in dollars per share)	$ 1.16	$ 0.64	$ 3.08	$ 1.70
Diluted earnings per share attributable to Tennant Company (in dollars per share)	$ 1.14	$ 0.63	$ 3.02	$ 1.68