Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Utility [Member]
Revenues from external customers	$ 261.7	$ 315.9	$ 574.5	$ 690.6
Intersegment revenues	84.9	81.0	171.7	156.1
Total revenues	346.6	396.9	746.2	846.7
Purchased power	69.3	104.4	154.2	222.2
Gross margin	208.7	203.4	427.8	436.8
Depreciation and amortization	36.1	33.4	70.8	66.5
Interest expense	9.4	9.7	19.0	19.4
Income tax expense (benefit)	15.6	15.5	32.9	42.5
Net income (loss)	31.3	30.8	69.4	83.5
Total Assets	3,488.5	3,525.7	3,488.5	3,525.7
Capital expenditures	56.3	48.4	109.5	90.8
Amortization of other assets
Fuel Costs	68.6	89.1	164.2	187.7
Competitive Retail [Member]
Revenues from external customers	109.9	102.0	222.0	196.0
Intersegment revenues
Total revenues	109.9	102.0	222.0	196.0
Purchased power	95.5	89.5	192.2	167.2
Gross margin	14.4	12.5	29.8	28.8
Depreciation and amortization	(0.1)		0.1	0.1
Interest expense	0.1	0.1	0.3	0.1
Income tax expense (benefit)	6.5	3.3	9.9	9.9
Net income (loss)	1.5	5.7	7.5	11.8
Total Assets	76.5	69.9	76.5	69.9
Capital expenditures	0.1		0.5
Amortization of other assets
Fuel Costs
Other Reportable Business Segment [Member]
Revenues from external customers	10.4	15.4	19.5	27.3
Intersegment revenues	0.8	1.0	1.7	2.0
Total revenues	11.2	16.4	21.2	29.3
Purchased power	0.4	0.7	0.4	1.1
Gross margin	(20.8)	12.7	(40.4)	24.0
Depreciation and amortization	(6.3)	1.7	(9.8)	3.6
Interest expense	23.1	7.9	43.1	15.1
Income tax expense (benefit)	(13.4)	(2.5)	(26.4)	(11.3)
Net income (loss)	(27.9)	(3.7)	(50.3)	(18.5)
Total Assets	2,406.4	2,472.1	2,406.4	2,472.1
Capital expenditures	0.1		0.5	0.6
Amortization of other assets	28.6		56.4
Fuel Costs	3.0	3.0	4.8	4.2
Adjustments and Eliminations [Member]
Revenues from external customers
Intersegment revenues	(85.7)	(82.0)	(173.4)	(158.1)
Total revenues	(85.7)	(82.0)	(173.4)	(158.1)
Purchased power	(84.9)	(81.0)	(171.7)	(156.1)
Gross margin	(0.8)	(1.0)	(1.7)	(2.0)
Depreciation and amortization
Interest expense	(0.2)	(0.1)	(0.4)	(0.1)
Income tax expense (benefit)
Net income (loss)		(1.1)		(1.6)
Total Assets
Capital expenditures
Amortization of other assets
Fuel Costs
Parent Company [Member]
Revenues from external customers	382.0	433.3	816.0	913.9
Intersegment revenues
Total revenues	382.0	433.3	816.0	913.9
Purchased power	80.3	113.6	175.1	234.4
Gross margin	201.5	227.6	415.5	487.6
Depreciation and amortization	29.7	35.1	61.1	70.2
Interest expense	32.4	17.6	62.0	34.5
Income tax expense (benefit)	8.7	16.3	16.4	41.1
Net income (loss)	4.9	31.7	26.6	75.2
Total Assets	5,971.4	6,067.7	5,971.4	6,067.7
Capital expenditures	56.5	48.4	110.5	91.4
Amortization of other assets	28.6		56.4
Fuel Costs	$ 71.6	$ 92.1	$ 169.0	$ 191.9