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Gains and losses on derivative instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative instruments [Line Items]
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$ 0	$ (5,849)
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	344	(2,772)
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	(6,154)
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	244	(2,843)
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	(1,422)
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	0	305
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	100	71
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	(159)
Commodity derivatives not designated as hedging instruments:
Derivative instruments [Line Items]
Amount of gain (loss) recognized in operating revenues, before tax	$ (6,712)	$ (4,410)

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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

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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

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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



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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



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