Derivative Instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Liabilities:
Fair value of liabilities	$ 1,862	$ 10,248
Other current assets
Assets:
Fair value of assets	1,290	222
Other deferred charges
Assets:
Fair value of assets	125	0
Other current liabilities
Liabilities:
Fair value of liabilities	577	2,305
Other deferred credits
Liabilities:
Fair value of liabilities	$ 2,700	$ 8,165

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
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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
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-Name Accounting Standards Codification
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