Debt (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Debt Instrument [Line Items]
Revolving credit facilities	1,000
Non-recourse debt and hybrid securities maximum limit	15.00%
Pre-tax charges related to tender			203.0
Alliant Energy Parent [Member]
Debt Instrument [Line Items]
Revolving credit facilities	300
IPL [Member]
Debt Instrument [Line Items]
Revolving credit facilities	300
WPL [Member]
Debt Instrument [Line Items]
Revolving credit facilities	400
2.5% Exchangeable Senior Notes Due 2030 [Member]
Debt Instrument [Line Items]
Tender offer and consent solicitation of debt instrument			5,940,960
Debt instrument interest rate			2.50%
Debt instrument maturity year	2030
Payments related to tender			$ 241
Senior notes tender and consents outstanding, number of notes		300
2.5% Exchangeable Senior Notes Due 2030 [Member] | Alliant Energy Corporation [Member]
Debt Instrument [Line Items]
Senior notes tender and consents outstanding, number of notes			5,940,660
4% Senior Notes Due 2014 [Member] | Alliant Energy Corporation [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.00%
Debt instrument maturity year	2014
5.06% Senior Secured Notes Due 2012 To 2024 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.06%
5.06% Senior Secured Notes Due 2012 To 2024 [Member] | Sheboygan Power LLC [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.06%
Minimum [Member] | IPL [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	2015
Minimum [Member] | WPL [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	2019
Minimum [Member] | 5.06% Senior Secured Notes Due 2012 To 2024 [Member] | Sheboygan Power LLC [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	2012
Maximum [Member] | IPL [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	2039
Maximum [Member] | WPL [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	2038
Maximum [Member] | 5.06% Senior Secured Notes Due 2012 To 2024 [Member] | Sheboygan Power LLC [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	2024