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Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Derivative Asset [Abstract] |  |
Gross asset | $ 134.8 | $ 138.8
Gross asset | 134.8 | 138.8
Pro forma net presentation, asset, total | 46.2 | 79.8
Counterparties with Offsetting Positions or Collateral [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 118.9 | 133.1
Pro forma net presentation, asset | 30.3 | 74.1
Counterparties without Offsetting Position [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 15.9 | 5.7
Pro forma net presentation, asset, total | 15.9 | 5.7
Current Assets from Risk Management Activities [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 85.5 | 103.3
Pro forma net presentation, asset, current | 20.1 | 59.5
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 81.1 | 99.8
Pro forma net presentation, asset | 15.7 | 56.0
Current Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 4.4 | 3.5
Pro forma net presentation, asset, current | 4.4 | 3.5
Long-Term Assets from Risk Management Activities [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 49.3 | 35.5
Pro forma net presentation, asset, noncurrent | 26.1 | 20.3
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 37.8 | 33.3
Pro forma net presentation, asset | 14.6 | 18.1
Long-Term Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 11.5 | 2.2
Pro forma net presentation, asset, noncurrent | $ 11.5 | $ 2.2

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