Jointly-Owned Electric Utility Plant (Details) $ in Millions	Dec. 31, 2018 USD ($)
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 1,789.4
Accumulated Provision for Depreciation	622.3
Construction Work in Progress	617.0
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	890.3
Accumulated Provision for Depreciation	333.2
Construction Work in Progress	68.7
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	899.1
Accumulated Provision for Depreciation	289.1
Construction Work in Progress	$ 548.3
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	48.00%
Electric Plant	$ 507.1
Accumulated Provision for Depreciation	161.7
Construction Work in Progress	$ 65.3
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	25.70%
Electric Plant	$ 187.3
Accumulated Provision for Depreciation	89.8
Construction Work in Progress	$ 1.2
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	28.00%
Electric Plant	$ 156.7
Accumulated Provision for Depreciation	57.8
Construction Work in Progress	$ 1.4
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	4.00%
Electric Plant	$ 39.2
Accumulated Provision for Depreciation	23.9
Construction Work in Progress	$ 0.8
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	52.50%
Electric Plant	$ 779.4
Accumulated Provision for Depreciation	247.9
Construction Work in Progress	$ 9.8
Forward Wind Energy Center [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	42.60%
Electric Plant	$ 119.7
Accumulated Provision for Depreciation	41.2
Construction Work in Progress	$ 0.1
West Riverside [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Ownership Interest %	91.80%	[1]
Electric Plant	$ 0.0	[1]
Accumulated Provision for Depreciation	0.0	[1]
Construction Work in Progress	$ 538.4	[1]

[1] In January 2018, certain electric cooperatives, which currently have wholesale power supply agreements with WPL, acquired a partial ownership interest in West Riverside.