Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Total revenues	$ 394.6	$ 434.0
Fuel Costs	88.6	97.4
Purchased power	95.3	94.8
Amortization of intangibles	1.8	27.8
Gross margin	208.9	214.0
Depreciation and amortization	31.8	31.4
Interest expense	30.5	29.6
Income tax expense (benefit)	6.0	7.7
Net income (loss)	19.9	21.7
Total Assets	4,274.3		4,247.3
Utility [Member]
Revenues from external customers	271.8	312.8
Intersegment revenues	104.7	86.8
Total revenues	376.5	399.6
Fuel Costs	88.1	95.6
Purchased power	94.1	84.9
Gross margin	194.3	219.1
Depreciation and amortization	33.6	34.7
Interest expense	9.3	9.6
Income tax expense (benefit)	9.6	17.3
Net income (loss)	30.2	38.1
Total Assets	3,439.6	3,464.2
Capital expenditures	33.6	53.2
Competitive Retail [Member]
Revenues from external customers	117.3	112.1
Total revenues	117.3	112.1
Purchased power	105.7	96.7
Gross margin	11.6	15.4
Depreciation and amortization	0.1	0.2
Interest expense	0.2	0.2
Income tax expense (benefit)	0.9	3.4
Net income (loss)	1.6	6.0
Total Assets	95.8	99.2
Capital expenditures		0.4
Other Reportable Business Segment [Member]
Revenues from external customers	5.5	9.1
Intersegment revenues	0.9	0.9
Total revenues	6.4	10.0
Fuel Costs	0.5	1.8
Purchased power	0.2
Amortization of intangibles	1.8	27.8
Gross margin	3.9	(19.6)
Depreciation and amortization	(1.9)	(3.5)
Interest expense	21.2	20.0
Income tax expense (benefit)	(4.5)	(13.0)
Net income (loss)	(11.9)	(22.4)
Total Assets	738.9	683.9
Capital expenditures	0.2	0.4
Adjustments and Eliminations [Member]
Intersegment revenues	(105.6)	(87.7)
Total revenues	(105.6)	(87.7)
Purchased power	(104.7)	(86.8)
Gross margin	(0.9)	(0.9)
Interest expense	(0.2)	(0.2)
Parent Company [Member]
Revenues from external customers	394.6	434.0
Total revenues	394.6	434.0
Fuel Costs	88.6	97.4
Purchased power	95.3	94.8
Amortization of intangibles	1.8	27.8
Gross margin	208.9	214.0
Depreciation and amortization	31.8	31.4
Interest expense	30.5	29.6
Income tax expense (benefit)	6.0	7.7
Net income (loss)	19.9	21.7
Total Assets	4,274.3	4,247.3
Capital expenditures	$ 33.8	$ 54.0