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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, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 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) | (912,000) | (1,016,000) | (1,763,000) | (1,957,000)
Not Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net loss recognized in income | (5,474,000) | 61,479,000 | (58,389,000) | 30,441,000
Not Designated as Hedging Instruments | Operating revenues |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net loss recognized in income | (58,000) | 585,000 | (346,000) | 483,000
Not Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net loss recognized in income | (5,416,000) | 60,894,000 | (58,043,000) | 29,958,000
Other comprehensive income | Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Gain (Loss) Recognized in OCI on Derivative Instruments (Effective Portion) | $ 11,000 | $ 208,000 | $ (84,000) | $ 60,000

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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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