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Risk Management Activities (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Jun. 24, 2016 | Dec. 31, 2015
Derivative [Line Items] |  |  |  |
Notional Amount of Forward-Starting Interest rate Swap Agreements | $ 1,500.0 |  | $ 500.0 |
Swap Forecasted Debt Issuance | 2018 |  |  |
Derivative, Notional Amount, Current | $ 250.0 |  |  |
Risk management assets |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net | 0.0 | $ 0.0 |  |
Derivative Liability, Fair Value, Gross Liability | 256.8 |  |  | $ 17.4
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months | $ 0.0 |  |  |

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