Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Intersegment revenues	$ 0.2			$ 0.2
Total revenues	479.2		441.2	1,329.6	1,210.7
Fuel Costs	85.1		99.7	235.9	274.0
Purchased power	153.7		113.1	466.2	282.6
Amortization of intangibles				0.9	5.3
Gross margin	240.1		226.6	626.6	648.8
Depreciation and amortization	34.5		33.9	103.7	99.0
Goodwill, Impairment Loss		135.8		135.8
Fixed asset impairment				11.5
Interest expense	33.1		31.0	95.8	91.1
Income tax expense (benefit)	(41.0)		11.3	29.7	20.8
Net income (loss)	98.4		33.2	(116.8)	76.0
Total Assets	3,542.6			3,542.6		3,721.5
Utility [Member]
External customer revenues	329.3		289.3	875.9	805.5
Intersegment revenues	125.6		123.8	376.6	336.0
Total revenues	454.9		413.1	1,252.5	1,141.5
Fuel Costs	84.5		96.7	227.4	269.6
Purchased power	152.4		110.4	457.3	276.7
Gross margin	218.0		206.0	567.8	595.2
Depreciation and amortization	36.4		35.8	108.2	104.5
Interest expense	9.4		10.4	25.5	29.7
Income tax expense (benefit)	13.1		13.2	23.1	29.2
Net income (loss)	53.2		40.9	76.5	101.4
Capital expenditures	25.6		28.3	78.6	95.1
Total Assets	3,245.3			3,245.3		3,313.1
Competitive Retail [Member]
External customer revenues	141.3		139.7	414.9	381.9
Total revenues	141.3		139.7	414.9	381.9
Purchased power	128.7		125.6	380.0	340.8
Gross margin	12.6		14.1	34.9	41.1
Depreciation and amortization	0.3		0.1	0.6	0.4
Interest expense	0.1		0.1	0.3	0.4
Income tax expense (benefit)	1.5		1.4	2.1	4.3
Net income (loss)	3.0		2.5	4.2	7.7
Capital expenditures	0.5			0.5
Total Assets	100.3			100.3		105.0
Other Reportable Business Segment [Member]
External customer revenues	8.6		12.2	38.8	23.3
Intersegment revenues	3.0		1.0	4.1	3.0
Total revenues	11.6		13.2	42.9	26.3
Fuel Costs	0.6		2.9	8.5	4.2
Purchased power	0.4		0.9	7.1	1.5
Amortization of intangibles	0.3		1.8	0.9	5.3
Gross margin	10.3		7.6	26.4	15.3
Depreciation and amortization	(2.2)		(2.0)	(5.1)	(5.9)
Goodwill, Impairment Loss				135.8
Fixed asset impairment				11.5
Interest expense	23.8		20.6	70.5	61.5
Income tax expense (benefit)	(55.6)		(3.3)	4.5	(12.7)
Net income (loss)	42.2		(10.2)	(197.5)	(33.1)
Capital expenditures	0.3		1.0	2.5	1.4
Total Assets	1,536.9			1,536.9		1,675.8
Adjustments and Eliminations [Member]
Intersegment revenues	(128.6)		(124.8)	(380.7)	(339.0)
Total revenues	(128.6)		(124.8)	(380.7)	(339.0)
Fuel Costs			0.1		0.2
Purchased power	(127.8)		(123.8)	(378.2)	(336.4)
Gross margin	(0.8)		(1.1)	(2.5)	(2.8)
Interest expense	(0.2)		(0.1)	(0.5)	(0.5)
Total Assets	(1,339.9)			(1,339.9)		(1,372.4)
Parent Company [Member]
External customer revenues	479.2		441.2	1,329.6	1,210.7
Total revenues	479.2		441.2	1,329.6	1,210.7
Fuel Costs	85.1		99.7	235.9	274.0
Purchased power	153.7		113.1	466.2	282.6
Amortization of intangibles	0.3		1.8	0.9	5.3
Gross margin	240.1		226.6	626.6	648.8
Depreciation and amortization	34.5		33.9	103.7	99.0
Goodwill, Impairment Loss				135.8
Fixed asset impairment				11.5
Interest expense	33.1		31.0	95.8	91.1
Income tax expense (benefit)	(41.0)		11.3	29.7	20.8
Net income (loss)	98.4		33.2	(116.8)	76.0
Capital expenditures	26.4		29.3	81.6	96.5
Total Assets	$ 3,542.6			$ 3,542.6		$ 3,721.5