Derivative Instruments and Hedging Activities (Gain or loss on derivative contracts) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on fair value adjustments	$ 27,650us-gaap_UnrealizedGainLossOnDerivatives	$ (2,730)us-gaap_UnrealizedGainLossOnDerivatives	$ 1,683us-gaap_UnrealizedGainLossOnDerivatives
Total gain (loss) on derivative instruments	31,736	(1,360)	1,717
Amount of gain reclassified from OCI into income			1,420
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) on derivative instruments	31,736	(1,360)	1,717
Natural Gas [Member] \| Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	(84)	(148)	34
Net gain (loss) on fair value adjustments	1,267	230	(241)
Total gain (loss) on derivative instruments	1,183	82	(207)
Crude Oil [Member] \| Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on settlements	4,170	1,518
Net gain (loss) on fair value adjustments	26,383	(2,960)	1,924
Total gain (loss) on derivative instruments	$ 30,553	$ (1,442)	$ 1,924

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

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-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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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=35706170&loc=SL5624171-113959

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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

-Topic 230

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-Section 45

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