Jointly-Owned Electric Utility Plant (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	$ 870.6
Accumulated Provision for Depreciation	466.4
Construction Work in Progress	485.6
Cost Of Removal Obligations Included In Regulatory Liabilities	45.9
IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	521.9
Accumulated Provision for Depreciation	255.4
Construction Work in Progress	214.4
Cost Of Removal Obligations Included In Regulatory Liabilities	33.5
WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
Electric Plant	348.7
Accumulated Provision for Depreciation	211.0
Construction Work in Progress	271.2
Cost Of Removal Obligations Included In Regulatory Liabilities	12.4
Ottumwa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
In-service Dates	1981
Ownership Interest %	48.00%
Electric Plant	246.8
Accumulated Provision for Depreciation	125.3
Construction Work in Progress	154.6
Cost Of Removal Obligations Included In Regulatory Liabilities	12.8
George Neal Unit 4 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
In-service Dates	1979
Ownership Interest %	25.70%
Electric Plant	180.4
Accumulated Provision for Depreciation	70.1
Construction Work in Progress	0.6
Cost Of Removal Obligations Included In Regulatory Liabilities	11.8
George Neal Unit 3 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
In-service Dates	1975
Ownership Interest %	28.00%
Electric Plant	59.5
Accumulated Provision for Depreciation	40.3
Construction Work in Progress	59.1
Cost Of Removal Obligations Included In Regulatory Liabilities	5.7
Louisa Unit 1 [Member] | IPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
In-service Dates	1983
Ownership Interest %	4.00%
Electric Plant	35.2
Accumulated Provision for Depreciation	19.7
Construction Work in Progress	0.1
Cost Of Removal Obligations Included In Regulatory Liabilities	3.2
Columbia Units 1-2 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
In-service Dates	1975-1978
Ownership Interest %	46.20%
Electric Plant	255.5
Accumulated Provision for Depreciation	159.4
Construction Work in Progress	270.5
Cost Of Removal Obligations Included In Regulatory Liabilities	10.1
Edgewater Unit 4 [Member] | WPL [Member]
Jointly Owned Electric Utility Plant [Line Items]
In-service Dates	1969
Ownership Interest %	68.20%
Electric Plant	93.2
Accumulated Provision for Depreciation	51.6
Construction Work in Progress	0.7
Cost Of Removal Obligations Included In Regulatory Liabilities	$ 2.3