Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Total revenues	$ 441.2	$ 471.7	$ 1,210.7	$ 1,287.7
Fuel Costs	99.7	112.7	274.0	279.0
Purchased power	113.1	90.7	282.6	265.8
Amortization of intangibles			5.3	71.2
Gross margin	226.6	244.1	648.8	671.7
Depreciation and amortization	33.9	33.1	99.0	95.6
Goodwill, Impairment Loss		1,850.0		1,850.0
Interest expense	31.0	31.1	91.1	93.1
Income tax expense (benefit)	11.3	20.2	20.8	40.3
Net income (loss)	33.2	(1,810.9)	76.0	(1,777.3)
Total Assets	4,506.1		4,506.1		4,247.3
Utility [Member]
Revenues from external customers	289.3	313.4	805.5	887.9
Intersegment revenues	123.8	113.4	336.0	285.1
Total revenues	413.1	426.8	1,141.5	1,173.0
Fuel Costs	96.7	108.1	269.6	272.3
Purchased power	110.4	79.9	276.7	234.1
Gross margin	206.0	238.8	595.2	666.6
Depreciation and amortization	35.8	36.5	104.5	107.3
Fixed asset impairment		80.8		80.8
Interest expense	10.4	10.0	29.7	29.0
Income tax expense (benefit)	13.2	6.5	29.2	39.4
Net income (loss)	40.9	(11.2)	101.4	58.3
Total Assets	3,837.5	3,464.2	3,837.5	3,464.2
Capital expenditures	28.3	52.2	95.1	161.7
Competitive Retail [Member]
Revenues from external customers	139.7	145.5	381.9	367.5
Total revenues	139.7	145.5	381.9	367.5
Purchased power	125.6	123.4	340.8	315.6
Gross margin	14.1	22.1	41.1	51.9
Depreciation and amortization	0.1	0.2	0.4	0.3
Interest expense	0.1	0.2	0.4	0.4
Income tax expense (benefit)	1.4	5.9	4.3	15.8
Net income (loss)	2.5	10.0	7.7	17.5
Total Assets	102.8	99.2	102.8	99.2
Capital expenditures				0.5
Other Reportable Business Segment [Member]
Revenues from external customers	12.2	12.8	23.3	32.3
Intersegment revenues	1.0	0.9	3.0	2.6
Total revenues	13.2	13.7	26.3	34.9
Fuel Costs	2.9	4.6	4.2	6.7
Purchased power	0.9	0.9	1.5	1.3
Amortization of intangibles	1.8	24.2	5.3	71.2
Gross margin	7.6	(16.0)	15.3	(44.3)
Depreciation and amortization	(2.0)	(3.6)	(5.9)	(12.0)
Goodwill, Impairment Loss		1,850.0		1,850.0
Interest expense	20.6	21.0	61.5	64.1
Income tax expense (benefit)	(3.3)	7.8	(12.7)	(14.9)
Net income (loss)	(10.2)	(1,809.7)	(33.1)	(1,853.1)
Total Assets	565.8	683.9	565.8	683.9
Capital expenditures	1.0	0.4	1.4	0.9
Adjustments and Eliminations [Member]
Intersegment revenues	(124.8)	(114.3)	(339.0)	(287.7)
Total revenues	(124.8)	(114.3)	(339.0)	(287.7)
Fuel Costs	0.1		0.2
Purchased power	(123.8)	(113.5)	(336.4)	(285.2)
Gross margin	(1.1)	(0.8)	(2.8)	(2.5)
Fixed asset impairment		(80.8)		(80.8)
Interest expense	(0.1)	(0.1)	(0.5)	(0.4)
Parent Company [Member]
Revenues from external customers	441.2	471.7	1,210.7	1,287.7
Total revenues	441.2	471.7	1,210.7	1,287.7
Fuel Costs	99.7	112.7	274.0	279.0
Purchased power	113.1	90.7	282.6	265.8
Amortization of intangibles	1.8	24.2	5.3	71.2
Gross margin	226.6	244.1	648.8	671.7
Depreciation and amortization	33.9	33.1	99.0	95.6
Goodwill, Impairment Loss		1,850.0		1,850.0
Interest expense	31.0	31.1	91.1	93.1
Income tax expense (benefit)	11.3	20.2	20.8	40.3
Net income (loss)	33.2	(1,810.9)	76.0	(1,777.3)
Total Assets	4,506.1	4,247.3	4,506.1	4,247.3
Capital expenditures	$ 29.3	$ 52.6	$ 96.5	$ 163.1