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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
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) | (1,148,000) | (1,298,000) | (2,910,000) | (3,255,000)
Not Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net loss recognized in income | (6,228,000) | (35,062,000) | (64,617,000) | (4,621,000)
Not Designated as Hedging Instruments | Operating revenues |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net loss recognized in income | (128,000) | 41,000 | (474,000) | 524,000
Not Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net loss recognized in income | (6,100,000) | (35,103,000) | (64,143,000) | (5,145,000)
Other comprehensive income | Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion) | $ 14,000 | $ (47,000) | $ (70,000) | $ 13,000

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959 + Details
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- Definition The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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