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Gains and losses on derivatives instruments (Details 2) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Derivative instruments [Line Items] |  |  |  |
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax | $ 254 | $ 25,773 | $ (5,594) | $ 22,506
Amount of (gain) loss reclassified from accumulated other comprehensive (effective portion), net of tax | (396) | (1,834) | (3,168) | (4,666)
Commodity derivatives designated as cash flow hedges [Member] |  |  |  |
Derivative instruments [Line Items] |  |  |  |
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax | 0 | 28,018 | (6,154) | 23,863
Amount of (gain) loss reclassified from accumulated other comprehensive (effective portion), net of tax | (871) | (1,840) | (3,714) | (4,687)
Amount of gain (loss) recognized in income statement (ineffective portion), before tax | 0 | 3,863 | (1,422) | (388)
Interest rate derivatives designated as cash flow hedges [Member] |  |  |  |
Derivative instruments [Line Items] |  |  |  |
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax | 254 | (2,245) | 560 | (1,357)
Amount of (gain) loss reclassified from accumulated other comprehensive (effective portion), net of tax | 475 | 6 | 546 | 21
Amount of gain (loss) recognized in income statement (ineffective portion), before tax | (610) | 0 | (769) | 0
Commodity derivatives not designated as hedging instruments: |  |  |  |
Derivative instruments [Line Items] |  |  |  |
Amount of gain (loss) recognized in operating revenues, before tax | $ 13,047 | $ 993 | $ 8,637 | $ 1,048

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- Definition The net gain (loss) recognized in earnings during the reporting period representing the amount of the cash flow hedges' ineffectiveness. + 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)(1) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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- Definition Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of price risk derivatives not designated 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=7476318&loc=SL5624171-113959 + Details
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- Definition Amount after tax of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e689-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph c -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41638-113959 + Details
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- Definition Amount after tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6441202&loc=d3e80720-113993 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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