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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in unrealized gain / (loss)	$ 1.6	$ 4.3	$ (5.6)
Realized gain / (loss)	1.6	(5.2)	(10.4)
Derivative, Gain (Loss) on Derivative, Net	3.2	(0.9)	(16.0)
Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)			0.4
Realized gain / (loss)			(0.3)
Derivative, Gain (Loss) on Derivative, Net			0.1
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.4)	0.3	0.3
Realized gain / (loss)	0.8	(0.6)	(0.2)
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.3)	0.1
Forward Contract Power [Member]
Change in unrealized gain / (loss)	1.9	4.0	(6.4)
Realized gain / (loss)	(0.7)	(7.2)	(9.8)
Derivative, Gain (Loss) on Derivative, Net	1.2	(3.2)	(16.2)
Natural Gas [Member]
Change in unrealized gain / (loss)	0.1	0.0	0.1
Realized gain / (loss)	1.5	2.6	(0.1)
Derivative, Gain (Loss) on Derivative, Net	1.6	2.6	0.0
Fuel [Member]
Derivative, Gain (Loss) on Derivative, Net			27.4
Fuel [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Fuel [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Fuel [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net			27.4
Fuel [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	4.4	16.4	(43.5)
Purchased Power [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.3)	0.1
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	2.4	14.1	(43.6)
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	1.6	2.6	0.0
Regulatory Asset [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.1
Regulatory Asset [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0.1
Regulatory Asset [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0
Regulatory Asset [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0
Regulatory Asset [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	$ 0.0	$ 0.0