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Risk Management Activities (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Derivative [Line Items] |  |  |
Limit of GCA Volumes | 20.00% |  |
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net | $ 0.0 | $ 0.0 | $ 0.0
Minimum |  |  |
Derivative [Line Items] |  |  |
Commodity Contract Length | 5 years |  |
Maximum |  |  |
Derivative [Line Items] |  |  |
Commodity Contract Length | 10 years |  |
Interest Rate Swap |  |  |
Derivative [Line Items] |  |  |
Derivative, Notional Amount | $ 500.0 |  | 750.0
Interest Rate Swap Settled |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | $ 0.0 |  | $ 46.2

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