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DERIVATIVE INSTRUMENTS: (Tables) | 9 Months Ended
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Sep. 30, 2021
DERIVATIVE INSTRUMENTS: |
Schedule of derivative instruments | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Call | ​ | Financial Swap | ​
Derivatives designated as hedging instruments under ASC 815 | ​ | Option | ​ | Cash Settlement | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commodity contracts | ​ | ​ | Natural gas | ​ | ​ | Natural gas | ​
Gas volume (MMBTUs) | ​ | ​ | 5,285,000 | ​ | ​ | 5,285,000 | ​
Fixed price ($) | ​ | ​ | 3.75 | ​ | ​ | 0.55 | ​
Total option premium (millions of $) | ​ |  | N/A | ​ | ​ | 2.9 | ​
Estimated fair value of assets (liabilities) as of September 30, 2021 (millions of $) | ​ |  | 12.1 | ​ | ​ | (2.9) | ​
Effect of derivative instruments on the consolidated Statement of Earnings (millions of $) | ​ |  | — | ​ | ​ | — | ​
(Favorable) unfavorable effect in OCI - net of deferred income taxes (millions of $) | ​ | ​ | (8.4) | ​ | ​ | 2.0 | ​

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- Definition Tabular disclosure of pertinent information about a derivative or group of derivatives on a disaggregated basis, such as for individual instruments, or small groups of similar instruments. May include a combination of the type of instrument, risks being hedged, notional amount, hedge designation, related hedged item, inception date, maturity date, or other relevant item. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624163-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41620-113959 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41638-113959 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41678-113959 Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41641-113959 Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579240-113959 + Details
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