Property, Plant and Equipment (Property, Plant and Equipment) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]	$ 0.0	$ 93.0
Total electric plant		12,663.6	12,296.9
Gas plant in service		1,387.6	1,244.0
Other plant in service		513.2	571.9
Accumulated depreciation		(4,314.6)	(4,283.1)
Net plant		10,249.8	9,829.7
Construction work in progress		1,774.8	962.2
Other, net		6.1	6.0
Total utility		12,030.7	10,797.9
Non-utility Generation, net	[2]	86.9	90.9
Corporate Services and other, net	[3]	344.8	345.7
Total non-utility and other		431.7	436.6
Total property, plant and equipment		12,462.4	11,234.5
Non-utility Generation, accumulated depreciation		54.5	50.5
Corporate Services and other, accumulated depreciation		167.5	285.6
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		6,800.6	6,655.3
Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		5,452.2	5,123.5
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		410.8	425.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early		0.0	0.0
Total electric plant		6,894.5	6,819.1
Gas plant in service		763.1	654.8
Other plant in service		322.4	333.4
Accumulated depreciation		(2,294.7)	(2,311.0)
Net plant		5,685.3	5,496.3
Construction work in progress		1,091.2	424.4
Other, net		5.0	5.5
Total utility		6,781.5	5,926.2
Total property, plant and equipment		6,781.5	5,926.2
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,610.4	3,715.9
IPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,023.7	2,820.9
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		260.4	282.3
WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]	0.0	93.0
Total electric plant		5,769.1	5,477.8
Gas plant in service		624.5	589.2
Other plant in service		190.8	238.5
Accumulated depreciation		(2,019.9)	(1,972.1)
Net plant		4,564.5	4,333.4
Leased Sheboygan Falls Energy Facility, net	[4]	38.1	46.2
Construction work in progress		683.6	537.8
Other, net		1.1	0.5
Total utility		5,287.3	4,917.9
Total property, plant and equipment		5,287.3	4,917.9
Leased Sheboygan Falls Energy Facility, accumulated depreciation		82.8	77.6
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,190.2	2,939.4
WPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		2,428.5	2,302.6
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		$ 150.4	$ 142.8

[1] In 2018, WPL retired Edgewater Unit 4 and reclassified the remaining net book value of this EGU from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets.

[2] Less accumulated depreciation of $54.5 million and $50.5 million for Alliant Energy as of December 31, 2018 and 2017, respectively.

[3] Less accumulated depreciation of $167.5 million and $285.6 million for Alliant Energy as of December 31, 2018 and 2017, respectively.

[4] Less accumulated amortization of $82.8 million and $77.6 million for WPL as of December 31, 2018 and 2017, respectively. The Sheboygan Falls Energy Facility is eliminated from WPL upon consolidation and is included in the “Non-utility Generation, net” line within Alliant Energy’s consolidated property, plant and equipment.