Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
External customer revenues	$ 298.1	$ 328.3	$ 622.0	$ 692.3
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	298.1	328.3	622.0	692.3
Depreciation and amortization	26.5	36.0	54.5	69.4
Fuel Costs	54.0	60.2	108.1	127.1
Depreciation and amortization			54.5	69.4
Impairment of Long-Lived Assets Held-for-use	0.0	235.5	66.4	235.5
Interest expense	27.4	26.0	54.3	52.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	8.8	(225.1)	(74.4)	(222.3)
Net income / (loss) from continuing operations	0.5	(136.8)	(51.2)	(134.6)
Discontinued operations, net of tax	0.0	0.0	0.0	29.6
Net income/ (loss)	0.5	(136.8)	(51.2)	(105.0)
Capital expenditures	25.0	41.4	66.4	79.1
Total assets	2,210.4		2,210.4		$ 2,419.2
Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
External customer revenues	167.9	184.7	357.7	388.9
Intersegment revenues	0.3	0.4	0.6	0.7
Total revenues	168.2	185.1	358.3	389.6
Depreciation and amortization	18.6	17.6	36.7	34.8
Impairment of Long-Lived Assets Held-for-use		0.0	0.0	0.0
Interest expense	7.8	5.6	15.2	11.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	15.1	29.8	40.1	64.0
Capital expenditures	19.4	18.1	45.7	42.2
Total assets	1,647.5		1,647.5		1,710.5
Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues	127.4	141.4	259.2	299.6
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	127.4	141.4	259.2	299.6
Depreciation and amortization	4.9	20.8	11.9	39.4
Impairment of Long-Lived Assets Held-for-use		857.1	66.3	857.1
Interest expense	0.2	0.1	0.2	0.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	13.2	(861.6)	(73.6)	(873.9)
Capital expenditures	5.1	23.3	19.2	36.3
Total assets	354.9		354.9		472.3
Other [Member]
Segment Reporting Information [Line Items]
External customer revenues	2.8	2.2	5.1	3.8
Intersegment revenues	1.3	0.9	2.7	2.2
Total revenues	4.1	3.1	7.8	6.0
Depreciation and amortization	3.0	(2.4)	5.9	(4.8)
Impairment of Long-Lived Assets Held-for-use		(621.6)	0.1	(621.6)
Interest expense	19.4	20.3	38.9	41.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(19.5)	606.7	(40.9)	587.6
Capital expenditures	0.5	0.0	1.5	0.6
Total assets	622.5		622.5		673.6
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0	0.0	0.0
Intersegment revenues	(1.6)	(1.3)	(3.3)	(2.9)
Total revenues	(1.6)	(1.3)	(3.3)	(2.9)
Depreciation and amortization	0.0	0.0	0.0	0.0
Impairment of Long-Lived Assets Held-for-use		0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	(0.1)
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	(414.5)		(414.5)		(437.2)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
External customer revenues	284.1	313.7	595.2	662.9
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	284.1	313.7	595.2	662.9
Depreciation and amortization	22.0	36.8	45.5	71.1
Fuel Costs	49.9	55.6	100.0	118.5
Depreciation and amortization			45.5	71.1
Impairment of Long-Lived Assets Held-for-use	0.0	857.1	66.3	857.1
Interest expense	7.8	5.4	15.4	10.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	26.3	(835.3)	(37.4)	(789.2)
Net income/ (loss)	16.1	(531.6)	(25.7)	(497.9)
Capital expenditures	23.5	36.0	57.6	71.8
Total assets	1,848.6		1,848.6		2,035.1
Subsidiaries [Member] | Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
External customer revenues	168.2	185.1	358.3	389.6
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	168.2	185.1	358.3	389.6
Depreciation and amortization	18.6	17.6	36.7	34.8
Impairment of Long-Lived Assets Held-for-use		0.0	0.0	0.0
Interest expense	7.8	5.3	15.2	10.5
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	15.1	30.1	40.1	64.5
Capital expenditures	19.4	18.1	45.7	42.2
Total assets	1,647.5		1,647.5		1,710.5
Subsidiaries [Member] | Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues	115.9	128.6	236.9	273.3
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	115.9	128.6	236.9	273.3
Depreciation and amortization	3.4	19.2	8.8	36.3
Impairment of Long-Lived Assets Held-for-use		857.1	66.3	857.1
Interest expense	0.0	0.1	0.2	0.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	11.2	(865.4)	(77.5)	(853.7)
Capital expenditures	4.1	17.9	11.9	29.6
Total assets	201.1		201.1		324.6
Subsidiaries [Member] | Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0	0.0	0.0
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Impairment of Long-Lived Assets Held-for-use		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	$ 0.0		$ 0.0		$ 0.0