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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, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
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) | (1,298,000) | (1,459,000) | (3,255,000) | (5,232,000)
Not Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net gain (loss) recognized in income | (35,062,000) | (50,349,000) | (4,621,000) | (84,654,000)
Not Designated as Hedging Instruments | Revenue |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net gain (loss) recognized in income | 41,000 | 560,000 | 524,000 | 445,000
Not Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net gain (loss) recognized in income | (35,103,000) | (50,909,000) | (5,145,000) | (85,099,000)
Other comprehensive income | Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Gain (loss) recognized in OCI on derivative instruments (effective portion) | $ (47,000) | $ (247,000) | $ 13,000 | $ (534,000)

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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=64837297&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=64837297&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=64837297&loc=SL5624171-113959 + Details
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