Derivative instruments (Amount included in statement of operations) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Not designated as hedging instruments \| Other income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 84	$ (128)	$ 120	$ (127)
Not designated as hedging instruments \| Foreign exchange gains (losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	896	0	(1,117)	0
Designated as hedging instruments \| Fair value hedge \| Foreign exchange contracts \| Foreign exchange gains (losses)
Derivatives designated as fair value hedges and related hedged item
Amount of loss recognized in income on derivative	0	(8,954)	0	(15,156)
Amount of gain on hedged item recognized in income attributable to risk being hedged	0	8,954	0	15,156
Amount of gain recognized in income on derivative (ineffective portion)	0	0	0	0
Designated as hedging instruments \| Cash flow hedge \| Interest rate swap contracts
Derivative instruments designated as a cash flow hedge
Amount of ineffective portion excluded from effectiveness testing	64	390	(694)	(411)
Designated as hedging instruments \| Cash flow hedge \| Interest rate swap contracts \| Other comprehensive income
Derivative instruments designated as a cash flow hedge
Amount of effective portion recognized in other comprehensive income	2,694	2,846	6,350	6,886
Designated as hedging instruments \| Cash flow hedge \| Interest rate swap contracts \| Finance expenses
Derivative instruments designated as a cash flow hedge
Amount of effective portion subsequently reclassified to earnings	$ (2,758)	$ (3,236)	$ (5,656)	$ (6,475)

Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying 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
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-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
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-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

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Amount of gain (loss) from the increase (decrease) in fair value of foreign currency derivatives and nonderivative instruments designated as fair value hedging instruments which were recognized in earnings, net of offsets by the gain (loss) on the hedged item to the extent that the fair value hedge was determined to be effective.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

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The net gain (loss) recognized in earnings during the period due to the ineffectiveness on foreign currency fair value hedges.

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Derivative Instruments, Gain (Loss) Recognized in Income Statement, Net [Abstract]

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