Financial instruments - Effects on Statement of Operations of Derivative Financial Instruments Not Designated as Hedges (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Fair Value of Financial Instruments [Line Items]
Total change in unrealized loss on derivative financial instruments	$ 334	$ (1,141)	$ 402	$ (198)
Gain (loss) on derivative instruments	(6,135)	(1,354)	(5,884)	(265)
Loss on derivative financial instruments	(6,135)	(1,354)	(5,884)	(265)
Not Designated as Hedging Instrument
Fair Value of Financial Instruments [Line Items]
Total change in unrealized loss on derivative financial instruments	(7,032)	(2,305)	(7,783)	(2,627)
Total realized loss on derivative financial instruments	(157)	196	149	359
Loss on derivative financial instruments not accounted for as hedges	(7,189)	(2,109)	(7,634)	(2,268)
Not Designated as Hedging Instrument \| Interest rate swaps
Fair Value of Financial Instruments [Line Items]
Total change in unrealized loss on derivative financial instruments	(4,680)	0	(4,680)	0
Not Designated as Hedging Instrument \| Energy derivative contracts
Fair Value of Financial Instruments [Line Items]
Total change in unrealized loss on derivative financial instruments	(2,352)	(2,305)	(3,103)	(2,627)
Total realized loss on derivative financial instruments	(157)	196	149	359
Designated as Hedging Instrument
Fair Value of Financial Instruments [Line Items]
Amortization of AOCI gains frozen as a result of hedge dedesignation	$ 1,054	$ 755	$ 1,750	$ 2,003

Derivative Instruments, Gain (Loss) Frozen due to Hedge Designation

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Fair Value of Financial Instruments [Line Items]

+ References

No definition available.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

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Gain (loss) incurred in hedging activities.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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-Topic 230
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