Quarterly Financial Data - Schedule of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Apr. 25, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Quarterly Financial Data [Line Items]
Recovery from a settlement with a vendor				$ 5,664
Gross profit (loss)		$ (3,465)	$ (1,059)	19,021	$ 2,551	$ (12,277)	$ (9,733)	$ 19,677	$ 7,318	$ 17,048	$ 4,985	$ 15,835
Unrealized gain (loss) on natural gas contracts		(1,743)	669	396	(527)
Net income (loss)		(21,692)	(20,402)	(365)	(19,452)	(27,714)	(30,794)	6,631	(11,540)	(61,911)	(63,417)	(72,226)
Charge associated with assets held for sale						(9,701)
Legal fees associated with Leidos matter		(572)	(901)	(955)	(3,287)	(3,843)	(3,330)	(1,496)	(932)
Interest expense associated with Global judgment	$ (900)	(80)	(80)	(79)	(1,327)					(51,115)	(46,389)	(43,064)
Benefit (provision) for income taxes		1,741	1,370	1,299	$ 339	15,108	483	5,733	$ (400)	$ 4,749	$ 20,924	$ (1,740)
Turnaround Expense [Member]
Schedule Of Quarterly Financial Data [Line Items]
Gross profit (loss)		$ (31)	$ (34)	$ (11)		$ (5,374)	$ (7,232)	$ (604)