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Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($) | 3 Months Ended | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 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) | (606,000) | (912,000) | (1,097,000) | (1,763,000)
Not Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | 3,043,000 | (5,474,000) | (32,265,000) | (58,389,000)
Not Designated as Hedging Instruments | Operating revenues |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | (341,000) | (58,000) | (1,560,000) | (346,000)
Not Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | 3,384,000 | (5,416,000) | (30,705,000) | (58,043,000)
Other comprehensive income | Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion) | $ 0 | $ 11,000 | $ 0 | $ (84,000)

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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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