Derivative Instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Liabilities:
Fair value of liabilities	$ 23,000,000	$ 13,154,000
Other current assets
Assets:
Fair value of assets		226,000
Other current liabilities
Liabilities:
Fair value of liabilities	10,709,000	2,066,000
Other deferred credits
Liabilities:
Fair value of liabilities	$ 12,291,000	$ 11,314,000

Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
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-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

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