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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Fair Value of Financial Instruments [Line Items]
Total change in unrealized gain (loss) on derivative financial instruments	$ 1,940	$ (9,144)	$ 2,179	$ (9,675)
Total realized gain (loss) on derivative financial instruments	0	433	0	156
Gain on derivative financial instruments	1,389	409	1,446	213
Gain (loss) on derivative instruments	1,389	842	1,446	369
Not Designated as Hedging Instrument
Fair Value of Financial Instruments [Line Items]
Total change in unrealized gain (loss) on derivative financial instruments	449	543	627	(19)
Total realized gain (loss) on derivative financial instruments	(549)	288	(681)	366
Gain (loss) on derivative financial instruments not accounted for as hedges	(100)	831	(54)	347
Not Designated as Hedging Instrument \| Energy derivative contracts
Fair Value of Financial Instruments [Line Items]
Total change in unrealized gain (loss) on derivative financial instruments	449	398	627	398
Total realized gain (loss) on derivative financial instruments	(549)	0	(681)	(207)
Not Designated as Hedging Instrument \| Foreign exchange forward
Fair Value of Financial Instruments [Line Items]
Total change in unrealized gain (loss) on derivative financial instruments	0	145	0	(417)
Total realized gain (loss) on derivative financial instruments	0	288	0	573
Designated as Hedging Instrument
Fair Value of Financial Instruments [Line Items]
Amortization of AOCI gains frozen as a result of hedge dedesignation	$ 1,489	$ 11	$ 1,500	$ 22

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
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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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