Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 50
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-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Section 45
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Section 50
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
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-Section 55
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-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Derivative Instruments - Fair value of derivative instruments not designated as hedging (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Interest rate options
Assets:
Fair value of assets	$ 5,000	$ 1,010,000
Interest rate options \| Other deferred charges
Assets:
Fair value of assets	5,000	1,010,000
Natural gas swaps
Assets:
Fair value of assets	2,645,000
Liabilities:
Fair Value of liabilities		22,848,000
Natural gas swaps \| Other deferred charges
Assets:
Fair value of assets	4,982,000
Natural gas swaps \| Other current liabilities
Assets:
Fair value of assets	$ 2,337,000
Liabilities:
Fair Value of liabilities		$ 22,848,000