Utility & Nonutility Plant - Utility Plant (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MW	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Original Cost	$ 7,015.4	$ 6,545.4
Utility Group
Property, Plant and Equipment [Line Items]
Original Cost	$ 7,015.4	6,545.4
Utility Group | Warrick Power Plant
Property, Plant and Equipment [Line Items]
Size of Unit 4 Warrick Power Plant (in megawatts) | MW	300
Utility Group | Gas utility plant
Property, Plant and Equipment [Line Items]
Original Cost	$ 3,969.6	$ 3,587.5
Depreciation Rates as a Percent of Original Cost	3.40%	3.40%
Utility Group | Electric utility plant
Property, Plant and Equipment [Line Items]
Original Cost	$ 2,833.5	$ 2,752.0
Depreciation Rates as a Percent of Original Cost	3.30%	3.30%
Utility Group | Common utility plant
Property, Plant and Equipment [Line Items]
Original Cost	$ 59.0	$ 56.3
Depreciation Rates as a Percent of Original Cost	3.20%	3.20%
Utility Group | Construction work in progress
Property, Plant and Equipment [Line Items]
Original Cost	$ 70.7	$ 63.0
Depreciation Rates as a Percent of Original Cost	0.00%	0.00%
Utility Group | Asset retirement obligations
Property, Plant and Equipment [Line Items]
Original Cost	$ 82.6	$ 86.6
Depreciation Rates as a Percent of Original Cost	0.00%	0.00%
Utility Group | SIGECO
Property, Plant and Equipment [Line Items]
SIGECO's share of cost of Warrick unit 4	$ 191.0
SIGECO's share of accumulated depreciation of Warrick unit 4	$ 119.7