Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2017 USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,692.0
Accumulated Provision for Depreciation	582.7
Construction Work in Progress	89.0
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	877.4
Accumulated Provision for Depreciation	305.5
Construction Work in Progress	42.0
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	814.6
Accumulated Provision for Depreciation	277.2
Construction Work in Progress	$ 47.0
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 501.8
Accumulated Provision for Depreciation	144.9
Construction Work in Progress	$ 37.3
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 187.2
Accumulated Provision for Depreciation	85.2
Construction Work in Progress	$ 0.8
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 150.6
Accumulated Provision for Depreciation	53.0
Construction Work in Progress	$ 2.5
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 37.8
Accumulated Provision for Depreciation	22.4
Construction Work in Progress	$ 1.4
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	50.10%
Electric Plant	$ 714.7
Accumulated Provision for Depreciation	216.8
Construction Work in Progress	$ 46.9
Edgewater Unit 4 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	68.20%
Electric Plant	$ 99.9
Accumulated Provision for Depreciation	60.4
Construction Work in Progress	$ 0.1