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Risk Management Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Risk Management Assets Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management assets | [1] | $ 13.7 | $ 0.6
Risk Management Assets Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management assets | [2] | 6.7 | 3.8
Risk Management Liabilities Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management liabilities |  | 96.9 | 12.6
Risk Management Liabilities Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management liabilities |  | 169.5 | 134.0
Interest Rate Risk | Risk Management Assets Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management assets | [1] | 0.0 | 0.0
Interest Rate Risk | Risk Management Assets Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management assets | [2] | 0.0 | 0.0
Interest Rate Risk | Risk Management Liabilities Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management liabilities |  | 92.1 | 0.0
Interest Rate Risk | Risk Management Liabilities Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management liabilities |  | 123.3 | 76.2
Commodity Price Risk Programs | Risk Management Assets Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management assets | [1] | 13.7 | 0.6
Commodity Price Risk Programs | Risk Management Assets Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management assets | [2] | 6.7 | 3.8
Commodity Price Risk Programs | Risk Management Liabilities Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management liabilities |  | 4.8 | 12.6
Commodity Price Risk Programs | Risk Management Liabilities Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Risk management liabilities |  | $ 46.2 | $ 57.8
[1] | Presented in "Prepayments and other" on the Condensed Consolidated Balance Sheets (unaudited).
[2] | Presented in "Deferred charges and other" on the Condensed Consolidated Balance Sheets (unaudited).

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