Derivative Instruments and Hedging Activities (Classification within the Condensed Consolidated Statements of Results of Operations or Balance Sheets of the Gains and Losses) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Change in unrealized gain / (loss)	$ 0.0	$ (0.2)
Realized gain / (loss)	0.2	(2.6)
Derivative, Gain (Loss) on Derivative, Net	0.2	(2.8)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.2	0.0
Realized gain / (loss)	0.2	0.2
Derivative, Gain (Loss) on Derivative, Net	0.4	0.2
Forward Contract Power [Member]
Change in unrealized gain / (loss)	(0.1)	(0.1)
Realized gain / (loss)	(0.2)	(2.6)
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(2.7)
Natural Gas [Member]
Change in unrealized gain / (loss)	(0.1)	(0.1)
Realized gain / (loss)	0.2	(0.2)
Derivative, Gain (Loss) on Derivative, Net	0.1	(0.3)
Revenue [Member]
Derivative, Gain (Loss) on Derivative, Net	(1.5)	(6.7)
Revenue [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Revenue [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(1.5)	(6.7)
Revenue [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	1.7	3.9
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.4	0.2
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	1.2	4.0
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 0.1	$ (0.3)