Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total contract assets	$ 111,800		$ 111,800				$ 60,100
Total advances on contracts	67,800		67,800				84,900
Revenue recognized	14,000	$ 20,000	44,000	$ 55,000
Contract with Customer, Liability, Revenue Recognized	14,000	20,000	44,000	55,000
Contract estimated forward loss provision for Harsco Rail Segment					$ 1,800	$ 45,100
Loss provision that exceeds accumulated contract costs	3,800		3,800				$ 4,400
Revenues	544,301	509,398	1,642,976	1,355,520
SBB [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 5,400	$ 11,800	$ 15,400	$ 29,500
Contract 2 [Member] | SBB [Member]
Disaggregation of Revenue [Line Items]
Percentage complete	81.00%		81.00%