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Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($) | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 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
Designated as Hedging Instruments | Fuel and purchased power |  |
Gains and losses from derivative instruments |  |
Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized) | (491,000) | (851,000)
Not Designated as Hedging Instruments |  |
Gains and losses from derivative instruments |  |
Net loss recognized in income | (35,308,000) | (52,915,000)
Not Designated as Hedging Instruments | Operating revenues |  |
Gains and losses from derivative instruments |  |
Net loss recognized in income | (1,219,000) | (288,000)
Not Designated as Hedging Instruments | Fuel and purchased power |  |
Gains and losses from derivative instruments |  |
Net loss recognized in income | (34,089,000) | (52,627,000)
Other comprehensive income | Designated as Hedging Instruments |  |
Gains and losses from derivative instruments |  |
Loss Recognized in OCI on Derivative Instruments (Effective Portion) | $ 0 | $ (96,000)

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- Definition The net gain (loss) reclassified from accumulated other comprehensive income into income on discontinuation of derivative instruments designated and qualifying as hedging instruments. + References No definition available. + Details
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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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