Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Total revenues	$ 390.1		$ 374.9	$ 850.4	$ 769.5
Fuel Costs	60.8		85.6	150.8	174.3
Purchased power	138.4		74.2	312.5	169.5
Amortization of intangibles				0.6	3.5
Gross margin	190.6		213.3	386.5	422.2
Depreciation and amortization	33.9		33.3	69.2	65.1
Goodwill, Impairment Loss		135.8		135.8
Fixed asset impairment				11.5
Interest expense	31.9		29.6	62.7	60.1
Income tax expense (benefit)	(28.1)		3.5	70.7	9.5
Net income (loss)	33.8		22.8	(215.2)	42.8
Total Assets	3,530.0			3,530.0		3,721.5
Utility [Member]
External customer revenues	254.0		244.4	546.6	516.2
Intersegment revenues	111.5		107.5	251.0	212.2
Total revenues	365.5		351.9	797.6	728.4
Fuel Costs	58.6		84.7	142.9	172.9
Purchased power	136.9		72.3	304.9	166.3
Gross margin	170.0		194.9	349.8	389.2
Depreciation and amortization	35.3		35.1	71.8	68.7
Interest expense	8.3		10.0	16.1	19.3
Income tax expense (benefit)	6.0		6.4	10.0	16.0
Net income (loss)	13.9		30.3	23.3	60.5
Capital expenditures	25.6		33.2	53.0	66.8
Total Assets	3,285.7		3,313.1	3,285.7	3,313.1
Competitive Retail [Member]
External customer revenues	125.2		124.9	273.6	242.2
Total revenues	125.2		124.9	273.6	242.2
Purchased power	111.1		109.5	251.3	215.2
Gross margin	14.1		15.4	22.3	27.0
Depreciation and amortization	0.2		0.2	0.3	0.3
Interest expense	0.1		0.1	0.2	0.3
Income tax expense (benefit)	1.3		2.0	0.6	2.9
Net income (loss)	2.6		3.6	1.2	5.2
Total Assets	98.7		105.0	98.7	105.0
Other Reportable Business Segment [Member]
External customer revenues	11.0		5.6	30.2	11.1
Intersegment revenues	0.1		1.0	1.1	1.9
Total revenues	11.1		6.6	31.3	13.0
Fuel Costs	2.2		0.9	7.9	1.4
Purchased power	1.3			6.7	0.4
Amortization of intangibles	0.3		1.8	0.6	3.5
Gross margin	7.3		3.9	16.1	7.7
Depreciation and amortization	(1.5)		(2.0)	(2.9)	(3.9)
Goodwill, Impairment Loss				135.8
Fixed asset impairment				11.5
Interest expense	23.6		19.7	46.7	40.9
Income tax expense (benefit)	(35.3)		(4.9)	60.1	(9.4)
Net income (loss)	17.3		(11.1)	(239.7)	(22.9)
Capital expenditures	1.2		0.2	2.2	0.4
Total Assets	1,500.2		1,675.8	1,500.2	1,675.8
Adjustments and Eliminations [Member]
External customer revenues	(0.1)
Intersegment revenues	(111.6)		(108.5)	(252.1)	(214.1)
Total revenues	(111.7)		(108.5)	(252.1)	(214.1)
Purchased power	(110.9)		(107.6)	(250.4)	(212.4)
Gross margin	(0.8)		(0.9)	(1.7)	(1.7)
Depreciation and amortization	(0.1)
Interest expense	(0.1)		(0.2)	(0.3)	(0.4)
Income tax expense (benefit)	(0.1)
Total Assets	(1,354.6)		(1,372.4)	(1,354.6)	(1,372.4)
Parent Company [Member]
External customer revenues	390.1		374.9	850.4	769.5
Total revenues	390.1		374.9	850.4	769.5
Fuel Costs	60.8		85.6	150.8	174.3
Purchased power	138.4		74.2	312.5	169.5
Amortization of intangibles	0.3		1.8	0.6	3.5
Gross margin	190.6		213.3	386.5	422.2
Depreciation and amortization	33.9		33.3	69.2	65.1
Goodwill, Impairment Loss				135.8
Fixed asset impairment				11.5
Interest expense	31.9		29.6	62.7	60.1
Income tax expense (benefit)	(28.1)		3.5	70.7	9.5
Net income (loss)	33.8		22.8	(215.2)	42.8
Capital expenditures	26.8		33.4	55.2	67.2
Total Assets	$ 3,530.0			$ 3,530.0		$ 3,721.5