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
	Mar. 31, 2021	Mar. 31, 2020
Net Income Attributable to Tennant Company	$ 25.7	$ 5.2
Basic - Weighted Average Shares Outstanding (in shares)	18,456,079	18,286,816
Effect of dilutive securities: Share-based compensation plans (in shares)	375,344	379,422
Diluted - Weighted Average Shares Outstanding (in shares)	18,831,423	18,666,238
Basic Earnings per Share (in dollars per share)	$ 1.39	$ 0.28
Diluted Earnings per Share (in dollars per share)	$ 1.37	$ 0.28