Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
External customer revenues	$ 372.9	$ 390.1	$ 867.4	$ 850.4
Total revenues	372.9	390.1	867.4	850.4
Fuel Costs	54.4	60.8	130.8	150.8
Purchased power	120.6	138.4	314.8	312.5
Amortization of intangibles		0.3	0.0	0.6
Gross margin	197.9	190.6	421.8	386.5
Depreciation and amortization	34.3	33.9	69.3	69.2
Goodwill impairment	0.0	0.0	0.0	135.8
Fixed asset impairment	0.0	0.0	0.0	11.5
Interest expense	30.9	31.9	61.4	62.7
Income tax expense (benefit)	0.5	(28.1)	13.2	70.7
Net income/ (loss)	21.7	33.8	50.4	(215.2)
Capital expenditures	31.1	26.8	64.8	55.2
Total assets	3,624.5		3,624.5		$ 3,577.8
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
External customer revenues	283.8	254.0	634.4	546.6
Intersegment revenues	68.3	111.5	179.0	251.0
Total revenues	352.1	365.5	813.4	797.6
Fuel Costs	50.6	58.6	119.9	142.9
Purchased power	120.1	136.9	309.8	304.9
Gross margin	181.4	170.0	383.7	349.8
Depreciation and amortization	34.2	35.3	68.9	71.8
Interest expense	9.0	8.3	17.7	16.1
Income tax expense (benefit)	9.4	6.0	24.2	10.0
Net income/ (loss)	23.9	13.9	60.4	23.3
Capital expenditures	30.2	25.6	63.3	53.0
Total assets	3,377.7		3,377.7		3,338.7
Operating Segments [Member] | Competitive Retail [Member]
Segment Reporting Information [Line Items]
External customer revenues	75.2	125.2	197.5	273.6
Total revenues	75.2	125.2	197.5	273.6
Purchased power	68.7	111.1	180.4	251.3
Gross margin	6.5	14.1	17.1	22.3
Depreciation and amortization	0.1	0.2	0.4	0.3
Interest expense	0.1	0.1	0.1	0.2
Income tax expense (benefit)	(5.5)	1.3	(4.2)	0.6
Net income/ (loss)	6.7	2.6	8.3	1.2
Capital expenditures	0.1		0.3
Total assets	49.8		49.8		94.9
Other [Member]
Segment Reporting Information [Line Items]
External customer revenues	13.9	11.0	35.5	30.2
Intersegment revenues	1.3	0.1	2.9	1.1
Total revenues	15.2	11.1	38.4	31.3
Fuel Costs	3.8	2.2	10.9	7.9
Purchased power	0.6	1.3	4.8	6.7
Amortization of intangibles		0.3		0.6
Gross margin	10.8	7.3	22.7	16.1
Depreciation and amortization	0.0	(1.5)		(2.9)
Goodwill impairment				135.8
Fixed asset impairment				11.5
Interest expense	21.8	23.6	43.7	46.7
Income tax expense (benefit)	(3.4)	(35.3)	(6.8)	60.1
Net income/ (loss)	(8.9)	17.3	(18.3)	(239.7)
Capital expenditures	0.8	1.2	1.2	2.2
Total assets	1,480.4		1,480.4		1,440.1
Adjustments and Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	(0.1)		0.0
Intersegment revenues	(69.6)	(111.6)	(181.9)	(252.1)
Total revenues	(69.6)	(111.7)	(181.9)	(252.1)
Purchased power	(68.8)	(110.9)	(180.2)	(250.4)
Gross margin	(0.8)	(0.8)	(1.7)	(1.7)
Depreciation and amortization	0.0	(0.1)		0.0
Interest expense	0.0	(0.1)	(0.1)	(0.3)
Income tax expense (benefit)	0.0	(0.1)		$ 0.0
Net income/ (loss)	0.0	$ 0.0
Total assets	$ (1,283.4)		$ (1,283.4)		$ (1,295.9)