Reconciliation of Basic and Diluted Shares - Reconciliation of Basic and Diluted Shares (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Income (loss) from continuing operations, net of tax	$ 8,696	$ (7,843)	$ 24,621	$ (27,375)
Weighted-average shares outstanding - basic (in shares)	79,287	79,000	79,214	78,916
Dilutive effect of stock-based compensation (in shares)	988	0	1,142	0
Weighted-average shares outstanding - diluted (in shares)	80,275	79,000	80,356	78,916
Earnings (loss) from continuing operations per common share, attributable to Harsco Corporation common stockholders:
Basic (in dollars per share)	$ 0.11	$ (0.10)	$ 0.31	$ (0.35)
Diluted (in dollars per share)	$ 0.11	$ (0.10)	$ 0.31	$ (0.35)