Property, Plant and Equipment (Property, Plant and Equipment) (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]	$ 0.0	$ 157.6
Total electric plant		10,444.9	10,365.1
Gas plant in service		1,018.3	946.2
Other plant in service		530.6	539.3
Accumulated depreciation	[1]	(3,939.6)	(3,923.1)
Net plant		8,054.2	7,927.5
Construction work in progress		897.5	479.1
Other, net		18.5	22.3
Total utility		8,970.2	8,428.9
Non-regulated Generation, net	[2]	229.3	240.1
Corporate Services and other, net	[3]	319.6	269.4
Total non-regulated and other		548.9	509.5
Total property, plant and equipment		9,519.1	8,938.4
Non-regulated Generation, accumulated depreciation		59.0	49.5
Corporate Services and other, accumulated depreciation		252.9	229.1
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		5,643.7	5,463.0
Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		4,489.9	4,435.4
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		311.3	309.1
IPL [Member]
Property, Plant and Equipment [Line Items]
Total electric plant		5,671.7	5,559.6
Gas plant in service		513.6	474.0
Other plant in service		296.0	298.8
Accumulated depreciation	[1]	(2,152.8)	(2,124.5)
Net plant		4,328.5	4,207.9
Construction work in progress		578.2	325.0
Other, net		18.4	21.8
Total utility		4,925.1	4,554.7
Total property, plant and equipment		4,925.1	4,554.7
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		3,011.6	2,872.4
IPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		2,447.9	2,471.7
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		212.2	215.5
WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	[1]	0.0	157.6
Total electric plant		4,773.2	4,805.5
Gas plant in service		504.7	472.2
Other plant in service		234.6	240.5
Accumulated depreciation	[1]	(1,786.8)	(1,798.6)
Net plant		3,725.7	3,719.6
Leased Sheboygan Falls Energy Facility, net	[4]	58.6	64.7
Construction work in progress		319.3	154.1
Other, net		0.1	0.5
Total utility		4,103.7	3,938.9
Total property, plant and equipment		4,103.7	3,938.9
Leased Sheboygan Falls Energy Facility, accumulated depreciation		65.2	59.1
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		2,632.1	2,590.6
WPL [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		2,042.0	1,963.7
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service		$ 99.1	$ 93.6

[1] In December 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3 and transferred the remaining net book value from property, plant and equipment to regulatory assets pursuant to orders from the PSCW and FERC. Refer to Note 2 for discussion of the recovery of the remaining net book value of these EGUs.

[2] Less accumulated depreciation of $59.0 million and $49.5 million for Alliant Energy as of December 31, 2015 and 2014, respectively.

[3] Less accumulated depreciation of $252.9 million and $229.1 million for Alliant Energy as of December 31, 2015 and 2014, respectively.

[4] Less accumulated amortization of $65.2 million and $59.1 million for WPL as of December 31, 2015 and 2014, respectively.