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Derivative Instruments and Hedging Activities - Pro Forma Impact Of Offsetting Assets (Details) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Derivative Asset [Abstract] |  |
Gross asset | $ 83.3 | $ 134.8
Gross asset | 83.3 | 134.8
Pro forma net presentation, asset, total | 17.8 | 46.2
Counterparties with Offsetting Positions or Collateral [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 75.5 | 118.9
Pro forma net presentation, asset | 10.0 | 30.3
Counterparties without Offsetting Position [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 7.8 | 15.9
Pro forma net presentation, asset, total | 7.8 | 15.9
Current Assets from Risk Management Activities [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 76.8 | 85.5
Pro forma net presentation, asset, current | 17.8 | 20.1
Current Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 69.0 | 81.1
Pro forma net presentation, asset | 10.0 | 15.7
Current Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 7.8 | 4.4
Pro forma net presentation, asset, current | 7.8 | 4.4
Long-Term Assets from Risk Management Activities [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | 6.5 | 49.3
Pro forma net presentation, asset, noncurrent |  | 26.1
Long-Term Assets from Risk Management Activities [Member] | Counterparties with Offsetting Positions or Collateral [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset | $ 6.5 | 37.8
Pro forma net presentation, asset |  | 14.6
Long-Term Assets from Risk Management Activities [Member] | Counterparties without Offsetting Position [Member] |  |
Derivative Asset [Abstract] |  |
Gross asset |  | 11.5
Pro forma net presentation, asset, noncurrent |  | $ 11.5

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