Derivative Instruments - Natural Gas Commodity Contracts Fair Value Hedges (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2016
Derivative [Line Items]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	$ (233)	$ (233)
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	681	681
Gain (Loss) on Fair Value Hedges Recognized in Earnings	448	448
Loss on Fair Value Hedge Ineffectiveness	(83)	(83)
Gain on Fair Value Hedge Ineffectiveness	531	531
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	$ 448	$ 448

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments 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 the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Amount of gain (loss)recognized in the income statement from the hedging ineffectiveness of a fair value hedge.

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Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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Amount of gain recognized in the income statement from the hedging ineffectiveness of a fair value hedge.

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Amount of recognized loss from the hedging ineffectiveness of a fair value hedge.

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