Commitments, Contingencies and Legal Proceedings - Additional Information (Detail) $ in Thousands	1 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) Operable_Unit	Dec. 31, 2019 USD ($)
Commitments Contingencies And Litigation [Line Items]
Number of operable units | Operable_Unit		5
Term of agreement with third party for monitoring and maintenance in OU1-OU4a		30 years
Difference recorded in escrow account		$ 500
Total amount available in escrow account		9,000
Reserve for Environmental liabilities, current portion		4,500	$ 9,000
Other Long-Term Liabilities [Member]
Commitments Contingencies And Litigation [Line Items]
Reserve for Environmental liabilities, remaining portion		$ 14,300
Minimum [Member]
Commitments Contingencies And Litigation [Line Items]
Period over which cash expenditure to be incurred for monitoring activities		30 years
Glatfelter Consent Decree [Member]
Commitments Contingencies And Litigation [Line Items]
Payments for governments unreimbursed past costs	$ 20,500		$ 20,500
OU2-5 [Member]
Commitments Contingencies And Litigation [Line Items]
Expected cleanup work continuation year		in 2020