Business Segments (Segment Financial Information) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Sep. 30, 2011	Mar. 31, 2011
Utility [Member]
Revenues from external customers	$ 312.8		$ 374.7
Intersegment revenues	86.8		75.1
Total revenues	399.6		449.8
Purchased power	84.9		117.8
Gross margin	219.1		233.4
Depreciation and amortization	34.7		33.1
Interest expense	9.6		9.7
Income tax expense (benefit)	17.3		27.0
Net income (loss)	38.1		52.7
Total Assets	3,501.6		3,525.7
Capital expenditures			42.4
Amortization of other assets
Fuel Costs	95.6		98.6
Competitive Retail [Member]
Revenues from external customers	112.1		94.0
Intersegment revenues
Total revenues	112.1		94.0
Purchased power	96.7		77.7
Gross margin	15.4		16.3
Depreciation and amortization	0.2		0.2
Interest expense	0.2
Income tax expense (benefit)	3.4		6.6
Net income (loss)	6.0		6.1
Total Assets	72.8		69.9
Capital expenditures
Amortization of other assets
Fuel Costs
Other Reportable Business Segment [Member]
Revenues from external customers	9.1		11.9
Intersegment revenues	0.9		1.0
Total revenues	10.0		12.9
Purchased power			0.4
Gross margin	(19.6)		11.4
Depreciation and amortization	(3.5)		1.8
Interest expense	20.0		7.2
Income tax expense (benefit)	(13.0)		(8.8)
Net income (loss)	(22.4)		(14.8)
Total Assets	2,482.4		2,511.9
Capital expenditures			0.6
Amortization of other assets	27.8
Fuel Costs	1.8		1.1
Adjustments and Eliminations [Member]
Revenues from external customers
Intersegment revenues	(87.7)		(76.1)
Total revenues	(87.7)		(76.1)
Purchased power	(86.8)		(75.1)
Gross margin	(0.9)		(1.0)
Depreciation and amortization
Interest expense
Income tax expense (benefit)
Net income (loss)			(0.5)
Total Assets
Capital expenditures
Amortization of other assets
Fuel Costs
Parent Company [Member]
Revenues from external customers	434.0		480.6
Intersegment revenues
Total revenues	434.0		480.6
Purchased power	94.8		120.8
Gross margin	214.0		260.1
Depreciation and amortization	31.4		35.1
Interest expense	29.8		16.9
Income tax expense (benefit)	7.7		24.8
Net income (loss)	21.7		43.5
Total Assets	6,056.8		6,107.5
Capital expenditures			43.0
Amortization of other assets	27.8
Fuel Costs	$ 97.4		$ 99.7