Earnings per Share - Number of Shares Outstanding for Calculation of Earnings Per Share (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income attributable to CECO Environmental Corp.	$ 1,181	$ 3,412
Denominator
Basic weighted-average shares outstanding	35,396,705	35,155,377
Common stock equivalents arising from stock options and restricted stock awards	377,000	240,000
Diluted weighted-average shares outstanding	35,774,208	35,394,865