Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Forward Contract Power [Member]
Beginning accumulated derivative gain / (loss) in AOCI	$ 0.4	$ (2.8)	$ (4.3)
Net gains / (losses) associated with current period hedging transactions	0.0	0.0	8.8
Ending accumulated derivative gain / (loss) in AOCI	0.0	0.4	(2.8)
Interest Rate Contract [Member]
Beginning accumulated derivative gain / (loss) in AOCI	16.6	17.5	17.4
Net gains / (losses) associated with current period hedging transactions	(1.0)	(0.1)	0.8
Ending accumulated derivative gain / (loss) in AOCI	14.5	16.6	17.5
Portion expected to be reclassified to earnings in the next twelve months	$ (1.1)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	8 months
Interest Expense [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0	0.0	0.0
Interest Expense [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(1.1)	(0.8)	(0.7)
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(0.4)	3.2	(7.3)
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0	$ 0.0	$ 0.0

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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