Utility & Nonutility Plant (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Utility & Nonutility Plant
Original Cost	$ 4,979.9	$ 4,791.7
Cost of Non-Utility plant, net of depreciation and amortization	550.8	488.3
Utility Group [Member] | Gas Utility Plant [Member]
Utility & Nonutility Plant
Original Cost	2,516.8	2,410.2
Depreciation Rates as a Percent of Original Cost (in hundredths)	3.50%	3.60%
Utility Group [Member] | Electric Utility Plant [Member]
Utility & Nonutility Plant
Original Cost	2,316.8	2,258.6
Depreciation Rates as a Percent of Original Cost (in hundredths)	3.30%	3.40%
Utility Group [Member] | Common Utility Plant [Member]
Utility & Nonutility Plant
Original Cost	51.6	49.7
Depreciation Rates as a Percent of Original Cost (in hundredths)	2.90%	3.10%
Utility Group [Member] | Construction Work in Progress [Member]
Utility & Nonutility Plant
Original Cost	94.7	73.2
Depreciation Rates as a Percent of Original Cost (in hundredths)	0.00%	0.00%
Utility Group [Member] | Warrick Power Plant [Member]
Utility & Nonutility Plant
Size of Unit 4 Warrick Power Plant (in megawatts)	300
Southern Indiana Gas And Electric Company's Share Of Cost Of Unit 4	182.6
Southern Indiana Gas And Electric Company's Share Of Accumulated Depreciation Of Unit 4	70.3
Nonutility Group [Member]
Utility & Nonutility Plant
Nonutility plant accumulated depreciation and amortization	418.5	385.5
Capitalized interest on nonutility plant construction projects	2.1	2.1	6.0
Nonutility Group [Member] | Coal Mine Development Costs and Equipment [Member]
Utility & Nonutility Plant
Cost of Non-Utility plant, net of depreciation and amortization	222.0	196.9
Nonutility Group [Member] | Computer Hardware and Software [Member]
Utility & Nonutility Plant
Cost of Non-Utility plant, net of depreciation and amortization	101.9	114.5
Nonutility Group [Member] | Land and Buildings [Member]
Utility & Nonutility Plant
Cost of Non-Utility plant, net of depreciation and amortization	112.0	112.8
Nonutility Group [Member] | Vehicles and Equipment [Member]
Utility & Nonutility Plant
Cost of Non-Utility plant, net of depreciation and amortization	92.5	46.9
Nonutility Group [Member] | All Other [Member]
Utility & Nonutility Plant
Cost of Non-Utility plant, net of depreciation and amortization	$ 22.4	$ 17.2