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Risk Management Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset |  | $ 24.0 | $ 21.1
Derivative Liability |  | 51.7 | 71.7
Risk Management Assets Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset | [1] | 1.1 | 14.5
Risk Management Assets Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset | [2] | 22.9 | 6.6
Risk Management Liabilities Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability |  | 5.0 | 43.2
Risk Management Liabilities Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability |  | 46.7 | 28.5
Interest rate risk programs | Risk Management Assets Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset |  | 0.0 | 14.0
Interest rate risk programs | Risk Management Assets Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset |  | 18.5 | 5.6
Interest rate risk programs | Risk Management Liabilities Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability |  | 0.0 | 38.6
Interest rate risk programs | Risk Management Liabilities Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability |  | 9.5 | 0.0
Commodity price risk programs | Risk Management Assets Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset |  | 1.1 | 0.5
Commodity price risk programs | Risk Management Assets Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset |  | 4.4 | 1.0
Commodity price risk programs | Risk Management Liabilities Current |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability |  | 5.0 | 4.6
Commodity price risk programs | Risk Management Liabilities Noncurrent |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability |  | $ 37.2 | $ 28.5
[1] | Presented in "Prepayments and other" on the Consolidated Balance Sheets.
[2] | Presented in "Deferred charges and other" on the Consolidated Balance Sheets.

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