Business Segments (Segment Financial Information) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Revenues from external customers	$ 511.8	$ 516.9	$ 1,451.4	$ 1,413.6
Intersegment revenues
Total revenues	511.8	516.9	1,451.4	1,413.6
Fuel	129.0	104.3	320.9	297.1
Purchased power	108.3	119.0	342.7	282.7
Gross margin	274.5	293.6	787.8	833.8
Depreciation and amortization	35.8	32.2	106.0	105.3
Interest expense	16.8	17.6	51.3	53.0
Income tax expense (benefit)	28.6	40.4	69.7	106.9
Net income (loss)	67.1	86.4	142.3	218.8
Cash capital expenditures	49.9	38.6	141.3	113.7
Total Assets	3,676.5		3,676.5		3,813.3
Utility [Member]
Revenues from external customers	376.6	411.1	1,092.9	1,183.0
Intersegment revenues	90.2	75.9	246.3	165.9
Total revenues	466.8	487.0	1,339.2	1,348.9
Fuel	124.0	97.4	311.7	286.5
Purchased power	95.6	116.4	317.8	279.3
Gross margin	247.2	273.2	709.7	783.1
Depreciation and amortization	33.8	30.4	100.3	98.4
Interest expense	9.3	9.4	28.7	27.9
Income tax expense (benefit)	26.8	39.4	69.3	104.6
Net income (loss)	63.9	83.2	147.4	214.7
Cash capital expenditures	49.1	38.8	139.9	112.3
Total Assets	3,419.7		3,419.7		3,475.4
Competitive Retail [Member]
Revenues from external customers	118.6	84.5	314.6	189.1
Intersegment revenues
Total revenues	118.6	84.5	314.6	189.1
Fuel
Purchased power	101.4	75.9	268.6	165.9
Gross margin	17.2	8.6	46.0	23.2
Depreciation and amortization	0.1		0.2	0.1
Interest expense	0.1		0.2
Income tax expense (benefit)	4.2	1.4	14.1	5.7
Net income (loss)	7.8	4.7	19.6	11.8
Cash capital expenditures
Total Assets	62.1		62.1		35.7
Other Reportable Business Segment [Member]
Revenues from external customers	16.6	21.3	43.9	41.5
Intersegment revenues	1.1	1.1	3.1	3.3
Total revenues	17.7	22.4	47.0	44.8
Fuel	5.0	6.9	9.2	10.6
Purchased power	1.5	2.6	2.6	3.4
Gross margin	11.2	12.9	35.2	30.8
Depreciation and amortization	1.9	1.8	5.5	6.8
Interest expense	7.6	8.3	22.7	25.2
Income tax expense (benefit)	(2.4)	(0.4)	(13.7)	(3.4)
Net income (loss)	(6.2)	(4.2)	(24.7)	(7.6)
Cash capital expenditures	0.8	(0.2)	1.4	1.4
Total Assets	1,702.2		1,702.2		1,828.8
Adjustments and Eliminations [Member]
Revenues from external customers
Intersegment revenues	(91.3)	(77.0)	(249.4)	(169.2)
Total revenues	(91.3)	(77.0)	(249.4)	(169.2)
Fuel
Purchased power	(90.2)	(75.9)	(246.3)	(165.9)
Gross margin	(1.1)	(1.1)	(3.1)	(3.3)
Depreciation and amortization
Interest expense	(0.2)	(0.1)	(0.3)	(0.1)
Income tax expense (benefit)
Net income (loss)	1.6	2.7		(0.1)
Cash capital expenditures
Total Assets	$ (1,507.5)		$ (1,507.5)		$ (1,526.6)