Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Total revenues	$ 460.3	$ 394.6
Fuel Costs	90.0	88.6
Purchased power	174.1	95.3
Amortization of intangibles	0.3	1.8
Gross margin	195.9	208.9
Depreciation and amortization	35.3	31.8
Goodwill, Impairment Loss	135.8
Fixed asset impairment	11.5
Interest expense	30.8	30.5
Income tax expense (benefit)	98.8	6.0
Net income (loss)	(249.0)	19.9
Total Assets	3,591.3		3,721.5
Utility [Member]
External customer revenues	292.6	271.8
Intersegment revenues	139.5	104.7
Total revenues	432.1	376.5
Fuel Costs	84.3	88.1
Purchased power	168.0	94.1
Gross margin	179.8	194.3
Depreciation and amortization	36.5	33.6
Interest expense	7.8	9.3
Income tax expense (benefit)	4.0	9.6
Net income (loss)	9.4	30.2
Capital expenditures	27.4	33.6
Total Assets	3,343.1	3,313.1
Competitive Retail [Member]
External customer revenues	148.4	117.3
Total revenues	148.4	117.3
Purchased power	140.2	105.7
Gross margin	8.2	11.6
Depreciation and amortization	0.1	0.1
Interest expense	0.1	0.2
Income tax expense (benefit)	(0.7)	0.9
Net income (loss)	(1.4)	1.6
Total Assets	105.7	105.0
Other Reportable Business Segment [Member]
External customer revenues	19.2	5.5
Intersegment revenues	1.0	0.9
Total revenues	20.2	6.4
Fuel Costs	5.7	0.5
Purchased power	5.4	0.2
Amortization of intangibles	0.3	1.8
Gross margin	8.8	3.9
Depreciation and amortization	(1.4)	(1.9)
Goodwill, Impairment Loss	135.8
Fixed asset impairment	11.5
Interest expense	23.1	21.2
Income tax expense (benefit)	95.4	(4.5)
Net income (loss)	(257.0)	(11.9)
Capital expenditures	1.0	0.2
Total Assets	1,531.8	1,675.8
Adjustments and Eliminations [Member]
External customer revenues	0.1
Intersegment revenues	(140.5)	(105.6)
Total revenues	(140.4)	(105.6)
Purchased power	(139.5)	(104.7)
Gross margin	(0.9)	(0.9)
Depreciation and amortization	0.1
Interest expense	(0.2)	(0.2)
Income tax expense (benefit)	0.1
Total Assets	(1,389.3)	(1,372.4)
Parent Company [Member]
External customer revenues	460.3	394.6
Total revenues	460.3	394.6
Fuel Costs	90.0	88.6
Purchased power	174.1	95.3
Amortization of intangibles	0.3	1.8
Gross margin	195.9	208.9
Depreciation and amortization	35.3	31.8
Goodwill, Impairment Loss	135.8
Fixed asset impairment	11.5
Interest expense	30.8	30.5
Income tax expense (benefit)	98.8	6.0
Net income (loss)	(249.0)	19.9
Capital expenditures	28.4	33.8
Total Assets	$ 3,591.3		$ 3,721.5