Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total contract assets	$ 79,100				$ 60,100
Total advances on contracts	72,500				84,900
Revenue recognized	17,000	$ 18,000
Contract with Customer, Liability, Revenue Recognized	17,000	18,000
Contract estimated forward loss provision for Harsco Rail Segment			$ 1,800	$ 45,100
Loss provision that exceeds accumulated contract costs	3,800				$ 4,400
Revenues	528,855	398,841
SBB [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 5,200	$ 10,100
Contract 2 [Member] | SBB [Member]
Disaggregation of Revenue [Line Items]
Percentage complete	74.00%