Commitments, Contingencies and Legal Proceedings - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Operable_Unit	Dec. 31, 2020 USD ($)
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	$ 1,600
Total amount available in escrow account	8,900
Reserve for Environmental liabilities, current portion	3,600	$ 3,700
Other Long-Term Liabilities [Member]
Commitments Contingencies And Litigation [Line Items]
Reserve for Environmental liabilities, remaining portion	$ 14,500
Minimum [Member]
Commitments Contingencies And Litigation [Line Items]
Period over which cash expenditure to be incurred for monitoring activities	30 years