Business Segments (Segment Financial Information) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Revenues from external customers	$ 444.9	$ 445.5	$ 939.6	$ 896.7
Intersegment revenues
Total revenues	444.9	445.5	939.6	896.7
Fuel	92.1	90.9	191.9	192.8
Purchased power	113.6	90.9	234.4	163.7
Gross margin	239.2	263.7	513.3	540.2
Depreciation and amortization	35.1	35.7	70.2	73.1
Interest expense	17.6	17.5	34.5	35.4
Income tax expense (benefit)	16.3	30.1	41.1	66.5
Net income (loss)	31.7	61.4	75.2	132.4
Cash capital expenditures	48.4	35.3	91.4	75.1
Total Assets	3,711.6		3,711.6		3,813.3
Utility [Member]
Revenues from external customers	327.6	371.2	716.3	771.9
Intersegment revenues	81.0	52.7	156.1	90.0
Total revenues	408.6	423.9	872.4	861.9
Fuel	89.1	88.5	187.7	189.1
Purchased power	104.4	90.3	222.2	162.9
Gross margin	215.1	245.1	462.5	509.9
Depreciation and amortization	33.4	33.2	66.5	68.0
Interest expense	9.7	9.1	19.4	18.5
Income tax expense (benefit)	15.5	28.4	42.5	65.2
Net income (loss)	30.8	59.4	83.5	131.5
Cash capital expenditures	48.4	34.1	90.8	73.5
Total Assets	3,435.0		3,435.0		3,475.4
Competitive Retail [Member]
Revenues from external customers	102.0	62.8	196.0	104.6
Intersegment revenues
Total revenues	102.0	62.8	196.0	104.6
Fuel
Purchased power	89.5	52.7	167.2	90.0
Gross margin	12.5	10.1	28.8	14.6
Depreciation and amortization		0.1	0.1	0.1
Interest expense	0.1		0.1
Income tax expense (benefit)	3.3	3.1	9.9	4.3
Net income (loss)	5.7	5.0	11.8	7.1
Cash capital expenditures
Total Assets	59.5		59.5		35.7
Other Reportable Business Segment [Member]
Revenues from external customers	15.3	11.5	27.3	20.2
Intersegment revenues	1.0	1.1	2.0	2.2
Total revenues	16.3	12.6	29.3	22.4
Fuel	3.0	2.4	4.2	3.7
Purchased power	0.7	0.6	1.1	0.8
Gross margin	12.6	9.6	24.0	17.9
Depreciation and amortization	1.7	2.4	3.6	5.0
Interest expense	7.9	8.4	15.1	16.9
Income tax expense (benefit)	(2.5)	(1.4)	(11.3)	(3.0)
Net income (loss)	(3.7)	(1.4)	(18.5)	(3.4)
Cash capital expenditures		1.2	0.6	1.6
Total Assets	1,698.2		1,698.2		1,828.8
Adjustments and Eliminations [Member]
Revenues from external customers
Intersegment revenues	(82.0)	(53.8)	(158.1)	(92.2)
Total revenues	(82.0)	(53.8)	(158.1)	(92.2)
Fuel
Purchased power	(81.0)	(52.7)	(156.1)	(90.0)
Gross margin	(1.0)	(1.1)	(2.0)	(2.2)
Depreciation and amortization
Interest expense	(0.1)		(0.1)
Income tax expense (benefit)
Net income (loss)	(1.1)	(1.6)	(1.6)	(2.8)
Cash capital expenditures
Total Assets	$ (1,481.1)		$ (1,481.1)		$ (1,526.6)