Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2016 USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,601.4
Accumulated Provision for Depreciation	532.4
Construction Work in Progress	64.8
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	861.8
Accumulated Provision for Depreciation	289.0
Construction Work in Progress	13.6
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	739.6
Accumulated Provision for Depreciation	243.4
Construction Work in Progress	$ 51.2
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 489.4
Accumulated Provision for Depreciation	137.3
Construction Work in Progress	$ 11.1
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 185.2
Accumulated Provision for Depreciation	80.8
Construction Work in Progress	$ 1.6
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 150.7
Accumulated Provision for Depreciation	49.1
Construction Work in Progress	$ 0.3
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 36.5
Accumulated Provision for Depreciation	21.8
Construction Work in Progress	$ 0.6
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	46.20%
Electric Plant	$ 640.2
Accumulated Provision for Depreciation	185.0
Construction Work in Progress	$ 51.0
Edgewater Unit 4 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	68.20%
Electric Plant	$ 99.4
Accumulated Provision for Depreciation	58.4
Construction Work in Progress	$ 0.2