Jointly-Owned Electric Utility Plant (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 769.2
Accumulated Provision for Depreciation	452.0
Construction Work in Progress	289.8
Cost Of Removal Obligations Included In Regulatory Liabilities	44.5
IPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	430.4
Accumulated Provision for Depreciation	244.9
Construction Work in Progress	155.9
Cost Of Removal Obligations Included In Regulatory Liabilities	32.1
WPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	338.8
Accumulated Provision for Depreciation	207.1
Construction Work in Progress	133.9
Cost Of Removal Obligations Included In Regulatory Liabilities	12.4
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
In-service Dates	1981
Ownership Interest %	48.00%
Plant in Service	239.2
Accumulated Provision for Depreciation	118.8
Construction Work in Progress	84.0
Cost Of Removal Obligations Included In Regulatory Liabilities	12.0
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
In-service Dates	1979
Ownership Interest %	25.70%
Plant in Service	97.1
Accumulated Provision for Depreciation	67.8
Construction Work in Progress	44.5
Cost Of Removal Obligations Included In Regulatory Liabilities	11.5
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
In-service Dates	1975
Ownership Interest %	28.00%
Plant in Service	59.1
Accumulated Provision for Depreciation	39.1
Construction Work in Progress	27.2
Cost Of Removal Obligations Included In Regulatory Liabilities	5.6
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
In-service Dates	1983
Ownership Interest %	4.00%
Plant in Service	35.0
Accumulated Provision for Depreciation	19.2
Construction Work in Progress	0.2
Cost Of Removal Obligations Included In Regulatory Liabilities	3.0
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
In-service Dates	1975-1978
Ownership Interest %	46.20%
Plant in Service	247.0
Accumulated Provision for Depreciation	157.9
Construction Work in Progress	133.5
Cost Of Removal Obligations Included In Regulatory Liabilities	10.2
Edgewater Unit 4 [Member] | WPL [Member]
Jointly Owned Utility Plant Interests [Line Items]
In-service Dates	1969
Ownership Interest %	68.20%
Plant in Service	91.8
Accumulated Provision for Depreciation	49.2
Construction Work in Progress	0.4
Cost Of Removal Obligations Included In Regulatory Liabilities	$ 2.2