Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
External customer revenues	$ 389.3	$ 414.1	$ 1,081.6	$ 1,281.5
Total revenues	389.3	414.1	1,081.6	1,281.5
Fuel Costs	78.9	71.4	206.0	202.2
Purchased power	111.7	145.4	330.5	460.2
Gross margin	198.7	197.3	545.1	619.1
Depreciation and amortization	30.9	34.8	100.3	104.1
Impairment of Long-Lived Assets Held-for-use	0.0	0.0	235.5	0.0
Interest expense	27.0	28.9	79.3	90.3
Income tax expense / (benefit)	12.7	0.3	(75.0)	13.5
Net income / (loss) from continuing operations	15.1	5.5	(119.5)	47.1
Discontinued operations, net of tax	0.0	3.1	29.6	11.9
Net income/ (loss)	15.1	8.6	(89.9)	59.0
Capital expenditures	30.7	28.7	109.8	93.5
Total assets	2,950.4		2,950.4		$ 3,324.7
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
External customer revenues	368.0	323.2	1,030.3	957.6
Intersegment revenues	0.4	66.0	1.0	245.0
Total revenues	368.4	389.2	1,031.3	1,202.6
Fuel Costs	71.0	69.0	189.5	188.9
Purchased power	110.0	142.5	328.0	452.3
Gross margin	187.4	177.7	513.8	561.4
Depreciation and amortization	24.1	34.6	95.2	103.5
Impairment of Long-Lived Assets Held-for-use			857.1
Interest expense	6.5	6.9	17.2	24.6
Income tax expense / (benefit)	19.7	0.8	(271.6)	25.0
Net income / (loss) from continuing operations			(467.8)
Discontinued operations, net of tax			0.0
Net income/ (loss)	30.1	15.5	(467.8)	75.9
Capital expenditures	26.5	27.9	98.3	91.2
Total assets	2,460.8		2,460.8		3,359.6
Operating Segments [Member] | Competitive Retail [Member]
Segment Reporting Information [Line Items]
External customer revenues		77.0		274.5
Intersegment revenues		0.0		0.0
Total revenues		77.0		274.5
Fuel Costs		0.0		0.0
Purchased power		66.6		247.0
Gross margin		10.4		27.5
Depreciation and amortization		0.2		0.6
Interest expense		0.0		0.1
Income tax expense / (benefit)		1.6		(2.6)
Net income/ (loss)		2.6		10.9
Capital expenditures		0.3		0.6
Total assets					0.0
Other [Member]
Segment Reporting Information [Line Items]
External customer revenues	21.3	13.9	51.3	49.4
Intersegment revenues	2.0	1.5	4.2	4.4
Total revenues	23.3	15.4	55.5	53.8
Fuel Costs	7.9	2.4	16.5	13.3
Purchased power	3.0	3.0	4.9	7.8
Gross margin	12.4	10.0	34.1	32.7
Depreciation and amortization	6.8	0.0	5.1	0.0
Impairment of Long-Lived Assets Held-for-use			(621.6)
Interest expense	20.3	22.1	61.6	65.8
Income tax expense / (benefit)	(7.0)	(2.1)	196.6	(8.9)
Net income / (loss) from continuing operations			348.3
Discontinued operations, net of tax			29.6
Net income/ (loss)	(15.0)	(9.5)	377.9	(27.8)
Capital expenditures	4.2	0.5	11.5	1.7
Total assets	1,303.8		1,303.8		1,304.5
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0	0.0	0.0
Intersegment revenues	(2.4)	(67.5)	(5.2)	(249.4)
Total revenues	(2.4)	(67.5)	(5.2)	(249.4)
Fuel Costs	0.0	0.0	0.0	0.0
Purchased power	(1.3)	(66.7)	(2.4)	(246.9)
Gross margin	(1.1)	(0.8)	(2.8)	(2.5)
Depreciation and amortization	0.0	0.0	0.0	0.0
Impairment of Long-Lived Assets Held-for-use			0.0
Interest expense	0.2	(0.1)	0.5	(0.2)
Income tax expense / (benefit)	0.0	0.0	0.0	0.0
Net income / (loss) from continuing operations			0.0
Discontinued operations, net of tax			0.0
Net income/ (loss)	0.0	0.0	0.0	0.0
Capital expenditures	0.0	$ 0.0	0.0	$ 0.0
Total assets	$ (814.2)		$ (814.2)		$ (1,339.4)