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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, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
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) | (538,000) | (606,000) | (974,000) | (1,097,000)
Not Designated as Hedging Instruments |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | (49,686,000) | 3,043,000 | (41,516,000) | (32,265,000)
Not Designated as Hedging Instruments | Operating revenues |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | 0 | (341,000) | 0 | (1,560,000)
Not Designated as Hedging Instruments | Fuel and purchased power |  |  |  |
Gains and losses from derivative instruments |  |  |  |
Net Gain (Loss) Recognized in Income | $ (49,686,000) | $ 3,384,000 | $ (41,516,000) | $ (30,705,000)

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