Derivatives (Effective Portion of Derivative Gain Loss) (Details) - Designated as Hedging Instrument [Member] - Cash Flow Hedging [Member] - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ (470)	$ (616)
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income (loss)	67	(540)
Natural Gas Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	(470)	(616)
Natural Gas Contracts \| Cost of Sales [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income (loss)	$ 67	$ (540)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

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
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624177-113959

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

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