Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	$ (364.0)		$ (312.7)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Regulatory liability recognized related to other postretirement benefit plans	2.7		3.3
Pension Plans, Defined Benefit [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Non-current assets	0		0
Other current liabilities	(2.4)		(4.6)
Pension and other benefit obligations	(239.5)		(179.4)
Net amount recognized at December 31	(241.9)		(184.0)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Net actuarial loss	533.4	[1]	494.8	[1]
Prior service credit	(7.2)	[1]	(6.9)	[1]
Net amount recognized at December 31	526.2	[1]	487.9	[1]
Other Postretirement Benefits Plans [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Non-current assets	3.5		1.3
Other current liabilities	(2.8)		0
Pension and other benefit obligations	(100.8)		(105.1)
Net amount recognized at December 31	(100.1)		(103.8)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Net actuarial loss	62.1	[1]	76.7	[1]
Prior service credit	(40.5)	[1]	(52.5)	[1]
Net amount recognized at December 31	21.6	[1]	24.2	[1]
IPL [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	(122.9)		(101.9)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Regulatory liability recognized related to other postretirement benefit plans	1.4		2.8
IPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	(66.1)		(45.8)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Net actuarial loss	149.7	[2]	141.1	[2]
Prior service credit	0.7	[2]	1.1	[2]
Net amount recognized at December 31	150.4	[2]	142.2	[2]
IPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	(17.2)		(22.8)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Net actuarial loss	32.0	[2]	41.0	[2]
Prior service credit	(21.3)	[2]	(27.6)	[2]
Net amount recognized at December 31	10.7	[2]	13.4	[2]
WPL [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Pension and other benefit obligations	(159.7)		(128.0)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Regulatory liability recognized related to other postretirement benefit plans	0.2		0
WPL [Member] | Pension Plans, Defined Benefit [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Non-current assets	0		0
Other current liabilities	0		0
Pension and other benefit obligations	(67.0)		(42.9)
Net amount recognized at December 31	(67.0)		(42.9)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Net actuarial loss	155.3	[3]	142.7	[3]
Prior service credit	1.9	[3]	2.4	[3]
Net amount recognized at December 31	157.2	[3]	145.1	[3]
WPL [Member] | Other Postretirement Benefits Plans [Member]
Amounts recognized on the Consolidated Balance Sheets consist of:
Non-current assets	3.5		1.3
Other current liabilities	(2.8)		0
Pension and other benefit obligations	(67.5)		(65.8)
Net amount recognized at December 31	(66.8)		(64.5)
Amounts recognized in Regulatory Assets, Regulatory Liabilities and AOCL consist of (b):
Net actuarial loss	24.3	[3]	27.8	[3]
Prior service credit	(13.4)	[3]	(17.3)	[3]
Net amount recognized at December 31	$ 10.9	[3]	$ 10.5	[3]

[1] Refer to Note 1(b) and Alliant Energy’s Consolidated Statements of Common Equity for amounts recognized in “Regulatory assets” and “AOCL,” respectively, on Alliant Energy’s Consolidated Balance Sheets. At December 31, 2012 and 2011, $2.7 million and $3.3 million, respectively, of regulatory liabilities were recognized related to Alliant Energy’s other postretirement benefits plans.

[2] Refer to Note 1(b) for amounts recognized in “Regulatory assets” on IPL’s Consolidated Balance Sheets. At December 31, 2012 and 2011, $1.4 million and $2.8 million, respectively, of regulatory liabilities were recognized related to IPL’s other postretirement benefits plans.

[3] Refer to Note 1(b) for amounts recognized in “Regulatory assets” on WPL’s Consolidated Balance Sheets. At December 31, 2012 and 2011, $0.2 million and $0, respectively, of regulatory liabilities were recognized related to WPL’s other postretirement benefits plans.