Derivative Commodity Instruments (Effect of Open and Closed Derivative Commodity Instruments Not Designated as Hedging Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Gain (Loss) on Derivative Instruments, Net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (loss) recognized in income on derivatives	$ 115,293	$ 313,408	$ (73,980)

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

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
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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