Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business			$ (13.2)	$ 0.0
External customer revenues	$ 207.7	$ 191.7	591.5	561.7
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	207.7	191.7	591.5	561.7
Depreciation and amortization	19.7	19.8	58.8	56.9
Fuel Costs	4.6	3.3	12.7	7.6
Depreciation and amortization			62.4	81.8
Impairment of Long-Lived Assets Held-for-use			2.8	66.4
Interest expense	22.6	27.8	74.4	82.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	19.8	(2.7)	13.7	(12.8)
Net income / (loss) from continuing operations	16.7	(1.0)	12.2	(7.8)
Discontinued operations, net of tax	4.3	22.9	29.2	(21.5)
Net income/ (loss)	21.0	21.9	41.4	(29.3)
Capital expenditures	25.1	29.2	75.8	95.6
Total assets	1,785.5		1,785.5		$ 2,049.2
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
External customer revenues	198.5	184.0	562.9	541.7
Intersegment revenues	0.2	0.2	0.6	0.8
Total revenues	198.7	184.2	563.5	542.5
Depreciation and amortization	19.1	19.6	56.5	56.3
Interest expense	5.8	7.9	20.5	23.5
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	37.5	20.0	73.9	60.1
Capital expenditures	20.5	20.6	65.0	66.3
Total assets	1,678.4		1,678.4		1,689.4
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
External customer revenues	9.2	7.7	28.6	20.0
Intersegment revenues	0.7	0.9	2.1	3.6
Total revenues	9.9	8.6	30.7	23.6
Depreciation and amortization	0.6	0.2	2.3	0.6
Interest expense	16.8	19.9	53.9	59.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(17.7)	(22.7)	(60.2)	(72.9)
Capital expenditures	4.6	8.6	10.8	29.3
Total assets	510.6		510.6		743.0
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0	0.0	0.0
Intersegment revenues	(0.9)	(1.1)	(2.7)	(4.4)
Total revenues	(0.9)	(1.1)	(2.7)	(4.4)
Depreciation and amortization	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	0.0	0.0	0.0	0.0
Capital expenditures	0.0	0.0	0.0	0.0
Total assets	(403.5)		(403.5)		(383.2)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Disposition of Business	0.0	0.0	(12.4)	0.0
Depreciation and amortization	19.1	19.6	56.5	56.3
Fuel Costs	0.1	0.0	1.7	0.0
Depreciation and amortization			56.5	68.2
Impairment of Long-Lived Assets Held-for-use			0.0	66.3
Interest expense	5.8	7.9	20.5	23.5
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	37.5	20.0	73.9	60.1
Net income / (loss) from continuing operations	31.2	20.9	61.9	49.2
Discontinued operations, net of tax	0.0	8.4	0.0	(45.6)
Net income/ (loss)	31.2	29.3	61.9	3.6
Total assets	$ 1,678.4		$ 1,678.4		$ 1,689.4
Subsidiaries [Member] | Generation [Member]
Segment Reporting Information [Line Items]
Discontinued operations, net of tax		$ 8.4		$ (45.6)