Derivatives and Fair Value Measurements (Details 7) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Sep. 30, 2010 Coal-fired generating plants facility	Dec. 31, 2011 Solar generating facilities facility	Dec. 31, 2011 Fair Value	Sep. 30, 2011 Fair Value	Dec. 31, 2010 Fair Value	Dec. 31, 2011 Fair Value Coal-fired generating plants	Dec. 31, 2011 Fair Value Biomass generating facilities	Dec. 31, 2011 Losses, end of period	Dec. 31, 2010 Losses, end of period	Dec. 31, 2011 Losses, end of period Biomass generating facilities	Dec. 31, 2011 Losses, end of period Solar generating facilities
Fair Value Assets Liabilities
Investment in CENG	$ 2,150.4	$ 2,991.1				$ 2,150.4	$ 2,970.4				$ 824.2	$ 2,275.0
Other investments:
UNE												143.4
Qualifying facilities - coal							36.7					50.0
Qualifying facilities - hydroelectric								14.8				8.4
Total other investments						47.1	36.7	14.8			66.8	201.8
Qualifying facilities									22.5	24.6	36.7		23.3	6.8
Total						2,197.5	3,007.1	14.8			891.0	2,476.8
Other-than-temporary impairment charge on equity method investments	$ 891.0	$ 8.4	$ 2,468.4
Number of coal-fired generating facilities in California				3
Number of solar generating facilities in California					3