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Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($) | 3 Months Ended | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges | $ 0 | $ 0 | $ 0 | $ 0
Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized) | (600,000) | (1,148,000) | (1,697,000) | (2,910,000)
Not Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | 3,234,000 | (6,228,000) | (29,032,000) | (64,617,000)
Not Designated as Hedging Instruments | Operating revenues |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | (1,029,000) | (128,000) | (2,590,000) | (474,000)
Not Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | 4,263,000 | (6,100,000) | (26,442,000) | (64,143,000)
Other comprehensive income | Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion) | $ 0 | $ 14,000 | $ 0 | $ (70,000)

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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624177-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + Details
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