Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues from external customers		$ 1,612.8	$ 1,763.0	$ 1,636.9
Total revenues		1,612.8	1,763.0	1,636.9
Fuel Costs		259.8	304.5	366.7
Purchased power		562.6	592.6	389.0
Amortization of intangibles		0.0	1.2	7.1
Gross margin		790.4	864.7	874.1
Depreciation and amortization		134.6	135.6	129.2
Depreciation and amortization		138.8	139.8	132.9
Goodwill impairment	[1]	317.0	135.8	306.3
Fixed asset impairment		0.0	11.5	26.2
Interest expense		118.3	126.6	124.0
Income tax expense / (benefit)		20.0	18.2	22.3
Net loss from continuing operations		(251.4)	57.2	(225.6)
Discontinued operations, net of tax		12.4	(131.8)	3.6
Net income (loss)		(239.0)	(74.6)	(222.0)
Cash capital expenditures		137.2	118.1	124.4
Total assets (end of year)		3,340.8	3,577.8	3,721.5
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		1,550.8	1,181.2	1,098.2
Intersegment revenues		1.5	487.1	453.3
Total revenues		1,552.3	1,668.3	1,551.5
Fuel Costs		244.7	314.9	362.5
Purchased power		555.7	582.4	381.9
Amortization of intangibles			0.0	0.0
Gross margin		751.9	771.0	807.1
Depreciation and amortization		138.2
Depreciation and amortization			144.8	140.2
Goodwill impairment		0.0	0.0	0.0
Fixed asset impairment		0.0	0.0	86.0
Interest expense		30.9	33.9	37.2
Income tax expense / (benefit)		35.1	39.7	18.6
Net loss from continuing operations		106.4
Discontinued operations, net of tax		0.0
Net income (loss)		106.4	115.0	83.6
Cash capital expenditures		127.0	114.2	122.1
Total assets (end of year)		3,365.8	3,338.7	3,313.1
Operating Segments [Member] | Competitive Retail [Member]
Segment Reporting Information [Line Items]
Revenues from external customers			533.6	511.6
Intersegment revenues			0.0	0.0
Total revenues			533.6	511.6
Fuel Costs			0.0	0.0
Purchased power			491.8	459.7
Amortization of intangibles			0.0	0.0
Gross margin			41.8	51.9
Depreciation and amortization			0.8	0.6
Goodwill impairment			0.0	0.0
Fixed asset impairment			0.0	0.0
Interest expense			0.5	0.5
Income tax expense / (benefit)			2.0	4.2
Net income (loss)			3.2	6.6
Cash capital expenditures			2.5	0.0
Total assets (end of year)			94.9	105.0
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		62.0	48.2	27.1
Intersegment revenues		4.2	5.5	4.0
Total revenues		66.2	53.7	31.1
Fuel Costs		15.1	(10.4)	4.2
Purchased power		8.9	7.5	1.1
Amortization of intangibles			1.2	7.1
Gross margin		42.2	55.4	18.7
Depreciation and amortization		(3.6)
Depreciation and amortization			(5.8)	(7.9)
Goodwill impairment		317.0	135.8	306.3
Fixed asset impairment		0.0	11.5	(59.8)
Interest expense		87.6	92.9	86.9
Income tax expense / (benefit)		(15.1)	(23.5)	(0.5)
Net loss from continuing operations		(357.8)
Discontinued operations, net of tax		12.4
Net income (loss)		(345.4)	(192.8)	(312.2)
Cash capital expenditures		10.2	1.4	2.3
Total assets (end of year)		1,314.4	1,440.1	1,675.8
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		0.0	0.0	0.0
Intersegment revenues		(5.7)	(492.6)	(457.3)
Total revenues		(5.7)	(492.6)	(457.3)
Fuel Costs		0.0	0.0	0.0
Purchased power		(2.0)	(489.1)	(453.7)
Amortization of intangibles			0.0	0.0
Gross margin		(3.7)	(3.5)	(3.6)
Depreciation and amortization		0.0
Depreciation and amortization			0.0	0.0
Goodwill impairment		0.0	0.0	0.0
Fixed asset impairment		0.0	0.0	0.0
Interest expense		(0.2)	(0.7)	(0.6)
Income tax expense / (benefit)		0.0	0.0	0.0
Net loss from continuing operations		0.0
Discontinued operations, net of tax		0.0
Net income (loss)		0.0	0.0	0.0
Cash capital expenditures		0.0	0.0	0.0
Total assets (end of year)		$ (1,339.4)	$ (1,295.9)	$ (1,372.4)

[1] Goodwill impairment of $135.8 million in 2014 has been reclassified to Discontinued operations in the Consolidated Statement of Operations.