Derivative Instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Liabilities:
Liabilities	$ 10,248	$ 32,256
Not designated as hedges \| Other current assets
Assets:
Assets	222	0
Not designated as hedges \| Other current liabilities
Liabilities:
Liabilities	2,305	12,898
Not designated as hedges \| Other deferred credits
Liabilities:
Liabilities	$ 8,165	$ 19,358

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
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-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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