Gains and losses on derivative instruments (Details 2) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	$ 92	$ 99
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	92	99
Not designated as hedging instrument [Member] \| Commodity derivatives [Member]
Derivative instruments [Line Items]
Amount of gain (loss) recognized in discontinued operations, before tax	$ 0	$ (11,208)

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).

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=66004124&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=64837297&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=66004124&loc=d3e637-108580

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration