Derivative Instruments (Income) (Details) (Not Designated as Hedging Instrument, USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Before-tax effect of derivative instruments not designated as hedging instruments on the consolidated statements of operations
Total activity for derivatives not designated as hedging instruments	$ 6,060	$ (737)	$ 4,305
Energy derivative contract
Before-tax effect of derivative instruments not designated as hedging instruments on the consolidated statements of operations
Unrealized gain (loss) on commodity contracts	6,478	(6,915)	(7,410)
Realized gain (loss) on commodity contracts	(235)	6,185	12,349
Interest Rate Swap
Before-tax effect of derivative instruments not designated as hedging instruments on the consolidated statements of operations
Unrealized gain (loss) on interest rate swap	312	258	(523)
Realized loss on interest rate swap	$ (495)	$ (265)	$ (111)

Realized gains or losses on interest rate swaps during the period.

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- Definition

Unrealized gains or losses on interest rate swaps during the period.

+ References

No definition available.

+ Details

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- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or 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 (e)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959

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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

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