Property, Plant and Equipment (Property, Plant and Equipment) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	157.6	[1]	$ 157.8	[1]
Total electric plant	10,365.1		9,415.7
Gas plant in service	946.2		909.9
Other plant in service	539.3		547.9
Accumulated depreciation	(3,923.1)		(3,726.2)
Net plant	7,927.5		7,147.3
Construction work in progress	479.1		677.9
Other, net	22.3		22.3
Total utility	8,428.9		7,847.5
Non-regulated Generation, net	240.1	[2]	249.4	[2]
Corporate Services and other, net	269.4	[3]	229.6	[3]
Total non-regulated and other	509.5		479.0
Total property, plant and equipment	8,938.4		8,326.5
Non-regulated Generation, accumulated depreciation	49.5		40.0
Corporate Services and other, accumulated depreciation	229.1		214.2
Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	5,463.0	[4]	4,792.0	[4]
Electric - distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	4,435.4		4,179.6
Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	309.1		286.3
IPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	0	[1]	0	[1]
Total electric plant	5,559.6		5,034.9
Gas plant in service	474.0		456.8
Other plant in service	298.8		302.8
Accumulated depreciation	(2,124.5)		(2,025.3)
Net plant	4,207.9		3,769.2
Construction work in progress	325.0		346.4
Other, net	21.8		21.2
Total utility	4,554.7		4,136.8
Total property, plant and equipment	4,554.7		4,136.8
IPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	2,872.4	[4]	2,513.2	[4]
IPL [Member] | Electric - distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	2,471.7		2,311.2
IPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	215.5		210.5
WPL [Member]
Property, Plant and Equipment [Line Items]
Electric plant anticipated to be retired early	157.6	[1]	157.8	[1]
Total electric plant	4,805.5		4,380.8
Gas plant in service	472.2		453.1
Other plant in service	240.5		245.1
Accumulated depreciation	(1,798.6)		(1,700.9)
Net plant	3,719.6		3,378.1
Leased Sheboygan Falls Energy Facility, net	64.7	[5]	70.9	[5]
Construction work in progress	154.1		331.5
Other, net	0.5		1.1
Total utility	3,938.9		3,781.6
Total property, plant and equipment	3,938.9		3,781.6
Leased Sheboygan Falls Energy Facility, accumulated depreciation	59.1		52.9
WPL [Member] | Generation [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	2,590.6	[4]	2,278.8	[4]
WPL [Member] | Electric - distribution [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	1,963.7		1,868.4
WPL [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Electric plant in service	93.6		$ 75.8
WPL [Member] | Edgewater Unit 3 and Nelson Dewey Units 1 and 2 [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years

[1] In 2013, WPL received approval from MISO to retire Edgewater Unit 3, and Nelson Dewey Units 1 and 2. WPL currently anticipates retiring these EGUs by December 31, 2015, contingent on completion of transmission network upgrades needed for system reliability. WPL is recovering the remaining net book value of these EGUs over a 10-year period beginning January 1, 2013 pursuant to a May 2012 PSCW order.

[2] Less accumulated depreciation of $49.5 million and $40.0 million for Alliant Energy as of December 31, 2014 and 2013, respectively.

[3] Less accumulated depreciation of $229.1 million and $214.2 million for Alliant Energy as of December 31, 2014 and 2013, respectively.

[4] Includes various emission controls projects placed in service in 2014, which are discussed in “Emission Controls Projects” below.

[5] Less accumulated amortization of $59.1 million and $52.9 million for WPL as of December 31, 2014 and 2013, respectively.