5. Derivatives - Schedule of Derivatives Not Designated as Hedging Instruments (Details) - Commodity Contracts [Member] - Non Designated Derivative Instruments [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Commodity contracts - Derivative assets	$ 3,017	$ 978
Commodity contracts - Derivative liabilities	$ 2,755	$ 4,115

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Name:	us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant