Business Segments (Segment Financial Information) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Total revenues	$ 494,500,000	$ 460,300,000
Fuel Costs	76,400,000	90,000,000
Purchased power	194,200,000	174,100,000
Amortization of intangibles		300,000
Gross margin	223,900,000	195,900,000
Depreciation and amortization	35,000,000	35,300,000
Goodwill, Impairment Loss		135,800,000
Fixed asset impairment		11,500,000
Interest expense	30,500,000	30,800,000
Income tax expense (benefit)	12,700,000	98,800,000
Net income (loss)	28,700,000	(249,000,000)
Total Assets	3,553,900,000		3,577,800,000
Utility [Member]
External customer revenues	350,600,000	292,600,000
Intersegment revenues	110,700,000	139,500,000
Total revenues	461,300,000	432,100,000
Fuel Costs	69,300,000	84,300,000
Purchased power	189,700,000	168,000,000
Gross margin	202,300,000	179,800,000
Depreciation and amortization	34,700,000	36,500,000
Interest expense	8,700,000	7,800,000
Income tax expense (benefit)	14,800,000	4,000,000
Net income (loss)	36,500,000	9,400,000
Capital expenditures	33,100,000	27,400,000
Total Assets	3,289,700,000		3,338,700,000
Competitive Retail [Member]
External customer revenues	122,300,000	148,400,000
Total revenues	122,300,000	148,400,000
Purchased power	111,700,000	140,200,000
Gross margin	10,600,000	8,200,000
Depreciation and amortization	300,000	100,000
Interest expense		100,000
Income tax expense (benefit)	1,300,000	(700,000)
Net income (loss)	1,600,000	(1,400,000)
Capital expenditures	200,000
Total Assets	72,300,000		94,900,000
Other Reportable Business Segment [Member]
External customer revenues	21,600,000	19,200,000
Intersegment revenues	1,600,000	1,000,000
Total revenues	23,200,000	20,200,000
Fuel Costs	7,100,000	5,700,000
Purchased power	4,200,000	5,400,000
Amortization of intangibles		300,000
Gross margin	11,900,000	8,800,000
Depreciation and amortization		(1,400,000)
Goodwill, Impairment Loss		135,800,000
Fixed asset impairment		11,500,000
Interest expense	21,900,000	23,100,000
Income tax expense (benefit)	(3,400,000)	95,400,000
Net income (loss)	(9,400,000)	(257,000,000)
Capital expenditures	400,000	1,000,000
Total Assets	1,476,100,000		1,440,100,000
Adjustments and Eliminations [Member]
External customer revenues		100,000
Intersegment revenues	(112,300,000)	(140,500,000)
Total revenues	(112,300,000)	(140,400,000)
Purchased power	(111,400,000)	(139,500,000)
Gross margin	(900,000)	(900,000)
Depreciation and amortization		100,000
Interest expense	(100,000)	(200,000)
Income tax expense (benefit)		100,000
Total Assets	(1,284,200,000)		(1,295,900,000)
Parent Company [Member]
External customer revenues	494,500,000	460,300,000
Total revenues	494,500,000	460,300,000
Fuel Costs	76,400,000	90,000,000
Purchased power	194,200,000	174,100,000
Amortization of intangibles		300,000
Gross margin	223,900,000	195,900,000
Depreciation and amortization	35,000,000	35,300,000
Goodwill, Impairment Loss		135,800,000
Fixed asset impairment		11,500,000
Interest expense	30,500,000	30,800,000
Income tax expense (benefit)	12,700,000	98,800,000
Net income (loss)	28,700,000	(249,000,000)
Capital expenditures	33,700,000	28,400,000
Total Assets	$ 3,553,900,000		$ 3,577,800,000