Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ 13.9	$ 15.3	$ 14.5	$ 16.6
Net gains / (losses) associated with current period hedging transactions	0.1	(0.2)	0.0	(1.0)
Ending accumulated derivative gain / (loss) in AOCI	13.7	14.7	13.7	14.7
Portion expected to be reclassified to earnings in the next twelve months	(0.1)		$ (0.1)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			0 months
Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI		0.4		0.4
Net gains / (losses) associated with current period hedging transactions		0.0		0.0
Ending accumulated derivative gain / (loss) in AOCI		0.0		0.0
Subsidiaries [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	(0.7)	(0.3)	$ (0.4)	0.6
Net gains / (losses) associated with current period hedging transactions	0.2	(0.1)	0.1	(0.9)
Ending accumulated derivative gain / (loss) in AOCI	(0.6)	(0.4)	(0.6)	(0.4)
Portion expected to be reclassified to earnings in the next twelve months	0.6		$ 0.6
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			0 months
Interest Expense [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.3)	(0.4)	$ (0.8)	(0.9)
Interest Expense [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings		0.0		0.0
Interest Expense [Member] \| Subsidiaries [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.1)	0.0	(0.3)	(0.1)
Discontinued Operations, Held-for-sale or Disposed of by Sale \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	$ 0.0	0.0	$ 0.0	0.0
Discontinued Operations, Held-for-sale or Disposed of by Sale \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings		$ (0.4)		$ (0.4)

Accumulated Derivative Gain/Loss in AOCI [Roll Forward]

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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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- Definition

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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