Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Total revenues	$ 374.9	$ 382.0	$ 769.5	$ 816.0
Fuel Costs	85.6	68.9	174.3	166.3
Purchased power	74.2	80.3	169.5	175.1
Amortization of intangibles			3.5	47.0
Gross margin	213.3	213.6	422.2	427.6
Depreciation and amortization	33.3	31.1	65.1	62.5
Interest expense	29.6	32.4	60.1	62.0
Income tax expense (benefit)	3.5	12.4	9.5	20.1
Net income (loss)	22.8	11.9	42.8	33.6
Total Assets	4,078.2		4,078.2		4,247.3
Utility [Member]
Revenues from external customers	244.4	261.7	516.2	574.5
Intersegment revenues	107.5	84.9	212.2	171.7
Total revenues	351.9	346.6	728.4	746.2
Fuel Costs	84.7	68.6	172.9	164.2
Purchased power	72.3	69.3	166.3	154.2
Gross margin	194.9	208.7	389.2	427.8
Depreciation and amortization	35.1	36.1	68.7	70.8
Interest expense	10.0	9.4	19.3	19.0
Income tax expense (benefit)	6.4	15.6	16.0	32.9
Net income (loss)	30.3	31.3	60.5	69.4
Total Assets	3,386.2	3,464.2	3,386.2	3,464.2
Capital expenditures	33.2	56.3	66.8	109.5
Competitive Retail [Member]
Revenues from external customers	124.9	109.9	242.2	222.0
Total revenues	124.9	109.9	242.2	222.0
Purchased power	109.5	95.5	215.2	192.2
Gross margin	15.4	14.4	27.0	29.8
Depreciation and amortization	0.2	(0.1)	0.3	0.1
Interest expense	0.1	0.1	0.3	0.3
Income tax expense (benefit)	2.0	6.5	2.9	9.9
Net income (loss)	3.6	1.5	5.2	7.5
Total Assets	97.4	99.2	97.4	99.2
Capital expenditures		0.1		0.5
Other Reportable Business Segment [Member]
Revenues from external customers	5.6	10.4	11.1	19.5
Intersegment revenues	1.0	0.8	1.9	1.7
Total revenues	6.6	11.2	13.0	21.2
Fuel Costs	0.9	0.3	1.4	2.1
Purchased power		0.4	0.4	0.4
Amortization of intangibles	1.8	19.2	3.5	47.0
Gross margin	3.9	(8.7)	7.7	(28.3)
Depreciation and amortization	(2.0)	(4.9)	(3.9)	(8.4)
Interest expense	19.7	23.1	40.9	43.1
Income tax expense (benefit)	(4.9)	(9.7)	(9.4)	(22.7)
Net income (loss)	(11.1)	(20.9)	(22.9)	(43.3)
Total Assets	594.6	683.9	594.6	683.9
Capital expenditures	0.2	0.1	0.4	0.5
Adjustments and Eliminations [Member]
Intersegment revenues	(108.5)	(85.7)	(214.1)	(173.4)
Total revenues	(108.5)	(85.7)	(214.1)	(173.4)
Purchased power	(107.6)	(84.9)	(212.4)	(171.7)
Gross margin	(0.9)	(0.8)	(1.7)	(1.7)
Interest expense	(0.2)	(0.2)	(0.4)	(0.4)
Parent Company [Member]
Revenues from external customers	374.9	382.0	769.5	816.0
Total revenues	374.9	382.0	769.5	816.0
Fuel Costs	85.6	68.9	174.3	166.3
Purchased power	74.2	80.3	169.5	175.1
Amortization of intangibles	1.8	19.2	3.5	47.0
Gross margin	213.3	213.6	422.2	427.6
Depreciation and amortization	33.3	31.1	65.1	62.5
Interest expense	29.6	32.4	60.1	62.0
Income tax expense (benefit)	3.5	12.4	9.5	20.1
Net income (loss)	22.8	11.9	42.8	33.6
Total Assets	4,078.2	4,247.3	4,078.2	4,247.3
Capital expenditures	$ 33.4	$ 56.5	$ 67.2	$ 110.5