Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total contract assets	$ 97,800		$ 97,800				$ 60,100
Total advances on contracts	70,000		70,000				84,900
Revenue recognized	13,000	$ 16,000	30,000	$ 34,000
Contract with Customer, Liability, Revenue Recognized	13,000	16,000	30,000	34,000
Contract estimated forward loss provision for Harsco Rail Segment					$ 1,800	$ 45,100
Loss provision that exceeds accumulated contract costs	3,300		3,300				$ 4,400
Revenues	569,820	447,281	1,098,675	846,122
SBB [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 4,800	$ 7,600	$ 10,000	$ 17,700
Contract 2 [Member] | SBB [Member]
Disaggregation of Revenue [Line Items]
Percentage complete	78.00%		78.00%