Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
External customer revenues	$ 364.0	$ 494.5
Total revenues	364.0	494.5
Fuel Costs	66.9	76.4
Purchased power	121.9	194.2
Gross margin	175.2	223.9
Depreciation and amortization	33.4	35.0
Interest expense	26.3	30.5
Income tax expense / (benefit)	0.6	12.7
Net income / (loss) from continuing operations	2.2	26.8
Discontinued operations, net of tax	29.6	1.9
Net income/ (loss)	31.8	28.7
Capital expenditures	37.7	33.7
Total assets	3,202.9		$ 3,324.7
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
External customer revenues	348.9	350.6
Intersegment revenues	0.3	110.7
Total revenues	349.2	461.3
Fuel Costs	62.9	69.3
Purchased power	121.3	189.7
Gross margin	165.0	202.3
Depreciation and amortization	34.3	34.7
Interest expense	5.3	8.7
Income tax expense / (benefit)	12.4	14.8
Net income / (loss) from continuing operations	33.7
Discontinued operations, net of tax	0.0
Net income/ (loss)	33.7	36.5
Capital expenditures	35.8	33.1
Total assets	3,318.1		3,359.6
Operating Segments [Member] | Competitive Retail [Member]
Segment Reporting Information [Line Items]
External customer revenues		122.3
Intersegment revenues		0.0
Total revenues		122.3
Fuel Costs		0.0
Purchased power		111.7
Gross margin		10.6
Depreciation and amortization		0.3
Interest expense		0.0
Income tax expense / (benefit)		1.3
Net income/ (loss)		1.6
Capital expenditures		0.2
Total assets			0.0
Other [Member]
Segment Reporting Information [Line Items]
External customer revenues	15.1	21.6
Intersegment revenues	1.3	1.6
Total revenues	16.4	23.2
Fuel Costs	4.0	7.1
Purchased power	1.2	4.2
Gross margin	11.2	11.9
Depreciation and amortization	(0.9)	0.0
Interest expense	21.0	21.9
Income tax expense / (benefit)	(11.8)	(3.4)
Net income / (loss) from continuing operations	(31.5)
Discontinued operations, net of tax	29.6
Net income/ (loss)	(1.9)	(9.4)
Capital expenditures	1.9	0.4
Total assets	1,222.3		1,304.5
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0
Intersegment revenues	(1.6)	(112.3)
Total revenues	(1.6)	(112.3)
Fuel Costs	0.0	0.0
Purchased power	(0.6)	(111.4)
Gross margin	(1.0)	(0.9)
Depreciation and amortization	0.0	0.0
Interest expense	0.0	(0.1)
Income tax expense / (benefit)	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
Capital expenditures	0.0	$ 0.0
Total assets	$ (1,337.5)		$ (1,339.4)