Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Goodwill, Impairment Loss	$ 1,850.0		$ 1,850.0
Utility [Member]
Revenues from external customers	313.4	362.3	887.9	1,052.9
Intersegment revenues	113.4	90.2	285.1	246.3
Total revenues	426.8	452.5	1,173.0	1,299.2
Fuel Costs	108.1	124.0	272.3	311.7
Purchased power	79.9	95.6	234.1	317.8
Gross margin	238.8	232.9	666.6	669.7
Depreciation and amortization	36.5	33.8	107.3	100.3
Impairment of Long-Lived Assets Held-for-use	80.8		80.8
Interest expense	10.0	9.3	29.0	28.7
Income tax expense (benefit)	6.5	26.8	39.4	69.3
Net income (loss)	(11.2)	63.9	58.3	147.4
Total Assets	3,386.6		3,386.6		3,538.3
Capital expenditures	52.2	49.1	161.7	139.9
Competitive Retail [Member]
Revenues from external customers	145.5	118.6	367.5	314.6
Total revenues	145.5	118.6	367.5	314.6
Purchased power	123.4	101.4	315.6	268.6
Gross margin	22.1	17.2	51.9	46.0
Depreciation and amortization	0.2	0.1	0.3	0.2
Interest expense	0.2	0.1	0.4	0.2
Income tax expense (benefit)	5.9	4.2	15.8	14.1
Net income (loss)	10.0	7.8	17.5	19.6
Total Assets	93.2		93.2		69.9
Capital expenditures			0.5
Other Reportable Business Segment [Member]
Revenues from external customers	12.8	16.7	32.3	44.0
Intersegment revenues	0.9	1.1	2.6	3.1
Total revenues	13.7	17.8	34.9	47.1
Fuel Costs	4.6	5.0	6.7	9.2
Purchased power	0.9	1.5	1.3	2.6
Amortization of intangibles	24.2		71.2
Gross margin	(16.0)	11.3	(44.3)	35.3
Depreciation and amortization	(3.6)	1.9	(12.0)	5.5
Goodwill, Impairment Loss	1,850.0		1,850.0
Interest expense	21.0	7.6	64.1	22.7
Income tax expense (benefit)	7.8	(2.4)	(14.9)	(13.7)
Net income (loss)	(1,809.7)	(6.2)	(1,853.1)	(24.7)
Total Assets	714.4		714.4		2,529.0
Capital expenditures	0.4	0.8	0.9	1.4
Adjustments and Eliminations [Member]
Intersegment revenues	(114.3)	(91.3)	(287.7)	(249.4)
Total revenues	(114.3)	(91.3)	(287.7)	(249.4)
Purchased power	(113.5)	(90.2)	(285.2)	(246.3)
Gross margin	(0.8)	(1.1)	(2.5)	(3.1)
Impairment of Long-Lived Assets Held-for-use	(80.8)		(80.8)
Interest expense	(0.1)	(0.2)	(0.4)	(0.3)
Net income (loss)		1.6
Parent Company [Member]
Revenues from external customers	471.7	497.6	1,287.7	1,411.5
Total revenues	471.7	497.6	1,287.7	1,411.5
Fuel Costs	112.7	129.0	279.0	320.9
Purchased power	90.7	108.3	265.8	342.7
Amortization of intangibles	24.2		71.2
Gross margin	244.1	260.3	671.7	747.9
Depreciation and amortization	33.1	35.8	95.6	106.0
Goodwill, Impairment Loss	1,850.0		1,850.0
Interest expense	31.1	16.8	93.1	51.3
Income tax expense (benefit)	20.2	28.6	40.3	69.7
Net income (loss)	(1,810.9)	67.1	(1,777.3)	142.3
Total Assets	4,194.2		4,194.2		6,137.2
Capital expenditures	$ 52.6	$ 49.9	$ 163.1	$ 141.3