Derivative Instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Liabilities:
Fair value of liabilities	$ 36,067	$ 32,256
Other current assets
Assets:
Fair value of assets	0	0
Other current liabilities
Liabilities:
Fair value of liabilities	17,905	12,898
Other deferred credits
Liabilities:
Fair value of liabilities	$ 18,162	$ 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
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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