Financial Instruments - Realized and unrealized gains and losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commodity Contract [Member] \| Not Designated as Hedging Instrument [Member]
Schedule of Realized and Unrealized Gains (Losses) from Commodity Derivatives
Derivative, Gain (Loss) on Derivative, Net	$ (1,247)	$ (3,987)	$ (13,477)	$ 4,886

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration