Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
External customer revenues	$ 323.9	$ 364.0
Intersegment revenues	0.0	0.0
Total revenues	323.9	364.0
Depreciation and amortization	28.0	33.4
Fuel Costs	54.1	66.9
Depreciation and amortization	28.0	33.4
Impairment of Long-Lived Assets Held-for-use	66.4	0.0
Interest expense	26.9	26.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	(83.2)	2.8
Net income / (loss) from continuing operations	(51.7)	2.2
Discontinued operations, net of tax	0.0	29.6
Net income/ (loss)	(51.7)	31.8
Capital expenditures	41.4	37.7
Total assets	2,264.2		$ 2,419.2
Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
External customer revenues	189.8	204.2
Intersegment revenues	0.3	0.3
Total revenues	190.1	204.5
Depreciation and amortization	18.1	17.2
Impairment of Long-Lived Assets Held-for-use	0.0
Interest expense	7.4	5.4
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	25.0	34.2
Capital expenditures	26.3	24.1
Total assets	1,680.9		1,710.5
Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues	131.8	158.2
Intersegment revenues	0.0	0.0
Total revenues	131.8	158.2
Depreciation and amortization	7.0	18.6
Impairment of Long-Lived Assets Held-for-use	66.3
Interest expense	0.0	0.1
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	(86.8)	(12.3)
Capital expenditures	14.1	13.0
Total assets	366.1		472.3
Other [Member]
Segment Reporting Information [Line Items]
External customer revenues	2.3	1.6
Intersegment revenues	1.4	1.3
Total revenues	3.7	2.9
Depreciation and amortization	2.9	(2.4)
Impairment of Long-Lived Assets Held-for-use	0.1
Interest expense	19.6	20.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	(21.4)	(19.1)
Capital expenditures	1.0	0.6
Total assets	633.6		673.6
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0
Intersegment revenues	(1.7)	(1.6)
Total revenues	(1.7)	(1.6)
Depreciation and amortization	0.0	0.0
Impairment of Long-Lived Assets Held-for-use	0.0
Interest expense	(0.1)	(0.1)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	0.0	0.0
Capital expenditures	0.0	0.0
Total assets	(416.4)		(437.2)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
External customer revenues	311.1	349.2
Intersegment revenues	0.0	0.0
Total revenues	311.1	349.2
Depreciation and amortization	23.5	34.3
Fuel Costs	50.1	62.9
Depreciation and amortization	23.5	34.3
Impairment of Long-Lived Assets Held-for-use	66.3	0.0
Interest expense	7.6	5.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	(63.7)	46.1
Net income/ (loss)	(41.8)	33.7
Capital expenditures	34.1	35.8
Total assets	1,889.9		2,035.1
Subsidiaries [Member] | Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
External customer revenues	190.1	204.5
Intersegment revenues	0.0	0.0
Total revenues	190.1	204.5
Depreciation and amortization	18.1	17.2
Impairment of Long-Lived Assets Held-for-use	0.0
Interest expense	7.4	5.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	25.0	34.4
Capital expenditures	26.3	24.1
Total assets	1,680.9		1,710.5
Subsidiaries [Member] | Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues	121.0	144.7
Intersegment revenues	0.0	0.0
Total revenues	121.0	144.7
Depreciation and amortization	5.4	17.1
Impairment of Long-Lived Assets Held-for-use	66.3
Interest expense	0.2	0.1
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	(88.7)	11.7
Capital expenditures	7.8	11.7
Total assets	209.0		324.6
Subsidiaries [Member] | Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0
Intersegment revenues	0.0	0.0
Total revenues	0.0	0.0
Depreciation and amortization	0.0	0.0
Impairment of Long-Lived Assets Held-for-use	0.0
Interest expense	0.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	0.0	0.0
Capital expenditures	0.0	$ 0.0
Total assets	$ 0.0		$ 0.0