Gains and losses on derivative instruments (Details 2) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss into discontinued operations (effective portion), net of tax	$ 0	$ (18)	$ 0	$ 140
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	100	100	299	299
Designated as Hedging Instrument [Member] \| Commodity derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss into discontinued operations (effective portion), net of tax	0	(18)	0	140
Designated as Hedging Instrument [Member] \| Interest rate derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	100	100	299	299
Not designated as hedging instrument [Member] \| Commodity derivatives [Member]
Derivative instruments [Line Items]
Amount of gain (loss) recognized in discontinued operations, before tax	$ 9,607	$ 28,755	$ (9,702)	$ 16,847

Amount after tax of reclassification adjustment from accumulated other comprehensive income for gain (loss) realized on derivative instruments included in income (loss) from discontinued operations, net of tax.

No definition available.

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+ References

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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

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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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 15

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e689-108580

Reference 2: 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=56946850&loc=d3e41638-113959

Reference 3: 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=36458714&loc=d3e637-108580

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