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Purchase commitments (Details 4) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Purchase commitments due |  |  |
2018 | $ 360,751 |  |
2019 | 215,005 |  |
2020 | 162,424 |  |
2021 | 135,334 |  |
2022 | 99,068 |  |
Thereafter | 773,820 |  |
Amounts purchased under various commitments | $ 516,100 | $ 539,300 | $ 842,100
Maximum [Member] |  |  |
Long-term Purchase Commitment [Line Items] |  |  |
Unrecorded Unconditional Purchase Obligation, Term | 43 years |  |

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- Definition Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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- Definition Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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- Definition Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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- Definition Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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- Definition Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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- Definition Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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- Definition The amount purchased during the period under an unrecorded unconditional purchase obligation (for example, under the take-or-pay or throughput contract). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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