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Financial instruments - Effects on Statement of Operations of Derivative Financial Instruments Not Designated as Hedges (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Fair Value of Financial Instruments [Line Items] |  |
Total change in unrealized gain on derivative financial instruments | $ (1,781) | $ 1,466
Total realized loss on derivative financial instruments | (1,115) | 12,558
Ineffective portion of derivative financial instruments accounted for as hedges | 33 | 27
Loss (gain) on derivative financial instruments | 636 | (1,918)
Gain (loss) on derivative instruments | (479) | 10,640
Not Designated as Hedging Instrument |  |
Fair Value of Financial Instruments [Line Items] |  |
Total change in unrealized gain on derivative financial instruments | (1,153) | (2,667)
Total realized loss on derivative financial instruments | 42 | 12,670
Loss (gain) on derivative financial instruments not accounted for as hedges | (1,111) | 10,003
Not Designated as Hedging Instrument | Interest rate swaps |  |
Fair Value of Financial Instruments [Line Items] |  |
Total realized loss on derivative financial instruments | 0 | (144)
Not Designated as Hedging Instrument | Energy derivative contracts |  |
Fair Value of Financial Instruments [Line Items] |  |
Total change in unrealized gain on derivative financial instruments | 77 | (79)
Total realized loss on derivative financial instruments | (73) | 553
Not Designated as Hedging Instrument | Foreign exchange forward |  |
Fair Value of Financial Instruments [Line Items] |  |
Total change in unrealized gain on derivative financial instruments | (1,230) | 297
Total realized loss on derivative financial instruments | 115 | 12,261
Not Designated as Hedging Instrument | Commodity contracts |  |
Fair Value of Financial Instruments [Line Items] |  |
Total change in unrealized gain on derivative financial instruments | 0 | (2,885)
Designated as Hedging Instrument |  |
Fair Value of Financial Instruments [Line Items] |  |
Ineffective portion of derivative financial instruments accounted for as hedges | $ 632 | $ 637

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1),(2) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 + Details
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- Definition Gain (loss) incurred in hedging activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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