Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
External customer revenues	$ 414.1	$ 479.2	$ 1,281.5	$ 1,329.6
Total revenues	414.1	479.2	1,281.5	1,329.6
Fuel Costs	71.4	85.1	202.2	235.9
Purchased power	145.4	153.7	460.2	466.2
Amortization of intangibles		0.3	0.0	0.9
Gross margin	197.3	240.1	619.1	626.6
Depreciation and amortization	34.8	34.5	104.1	103.7
Goodwill impairment	0.0	0.0	0.0	135.8
Fixed asset impairment	0.0	0.0	0.0	11.5
Interest expense	28.9	33.1	90.3	95.8
Income tax expense (benefit)	0.3	(41.0)	13.5	29.7
Net income/ (loss)	8.6	98.4	59.0	(116.8)
Capital expenditures	28.7	26.4	93.5	81.6
Total assets	3,489.4		3,489.4		$ 3,577.8
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
External customer revenues	323.2	329.3	957.6	875.9
Intersegment revenues	66.0	125.6	245.0	376.6
Total revenues	389.2	454.9	1,202.6	1,252.5
Fuel Costs	69.0	84.5	188.9	227.4
Purchased power	142.5	152.4	452.3	457.3
Amortization of intangibles		0.0		0.0
Gross margin	177.7	218.0	561.4	567.8
Depreciation and amortization	34.6	36.4	103.5	108.2
Goodwill impairment				0.0
Fixed asset impairment				0.0
Interest expense	6.9	9.4	24.6	25.5
Income tax expense (benefit)	0.8	13.1	25.0	23.1
Net income/ (loss)	15.5	53.2	75.9	76.5
Capital expenditures	27.9	25.6	91.2	78.6
Total assets	3,239.9		3,239.9		3,338.7
Operating Segments [Member] | Competitive Retail [Member]
Segment Reporting Information [Line Items]
External customer revenues	77.0	141.3	274.5	414.9
Intersegment revenues	0.0	0.0	0.0	0.0
Total revenues	77.0	141.3	274.5	414.9
Fuel Costs	0.0	0.0	0.0	0.0
Purchased power	66.6	128.7	247.0	380.0
Amortization of intangibles		0.0		0.0
Gross margin	10.4	12.6	27.5	34.9
Depreciation and amortization	0.2	0.3	0.6	0.6
Goodwill impairment				0.0
Fixed asset impairment				0.0
Interest expense	0.0	0.1	0.1	0.3
Income tax expense (benefit)	1.6	1.5	(2.6)	2.1
Net income/ (loss)	2.6	3.0	10.9	4.2
Capital expenditures	0.3	0.5	0.6	0.5
Total assets	44.5		44.5		94.9
Other [Member]
Segment Reporting Information [Line Items]
External customer revenues	13.9	8.6	49.4	38.8
Intersegment revenues	1.5	3.0	4.4	4.1
Total revenues	15.4	11.6	53.8	42.9
Fuel Costs	2.4	0.6	13.3	8.5
Purchased power	3.0	0.4	7.8	7.1
Amortization of intangibles		0.3		0.9
Gross margin	10.0	10.3	32.7	26.4
Depreciation and amortization	0.0	(2.2)		(5.1)
Goodwill impairment				135.8
Fixed asset impairment				11.5
Interest expense	22.1	23.8	65.8	70.5
Income tax expense (benefit)	(2.1)	(55.6)	(8.9)	4.5
Net income/ (loss)	(9.5)	42.2	(27.8)	(197.5)
Capital expenditures	0.5	0.3	1.7	2.5
Total assets	1,472.8		1,472.8		1,440.1
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0	0.0	0.0
Intersegment revenues	(67.5)	(128.6)	(249.4)	(380.7)
Total revenues	(67.5)	(128.6)	(249.4)	(380.7)
Fuel Costs	0.0	0.0	0.0	0.0
Purchased power	(66.7)	(127.8)	(246.9)	(378.2)
Amortization of intangibles		0.0		0.0
Gross margin	(0.8)	(0.8)	(2.5)	(2.5)
Depreciation and amortization	0.0	0.0	0.0	0.0
Goodwill impairment				0.0
Fixed asset impairment				0.0
Interest expense	(0.1)	(0.2)	(0.2)	(0.5)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Net income/ (loss)	0.0	0.0	0.0	0.0
Capital expenditures	0.0	$ 0.0	0.0	$ 0.0
Total assets	$ (1,267.8)		$ (1,267.8)		$ (1,295.9)