Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Alliant Energy Corporation [Member]
Assets:
Money market funds, at carrying value	$ 13.6	$ 0
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	3,598.7
Cumulative preferred stock of subsidiaries	215.6
IPL [Member]
Assets:
Money market funds, at carrying value	1.1	0
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,696.1
Cumulative preferred stock	154.8
WPL [Member]
Assets:
Money market funds, at carrying value	12.5	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,492.4
Cumulative preferred stock	60.8
Commodity Contracts [Member] | Alliant Energy Corporation [Member]
Assets:
Derivative assets	46.5	15.7
Capitalization and liabilities:
Derivatives	57.3	78.0
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	28.1	10.6
Capitalization and liabilities:
Derivatives	22.8	33.6
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	18.4	5.1
Capitalization and liabilities:
Derivatives	34.5	44.4
Level 1 [Member] | Alliant Energy Corporation [Member]
Assets:
Money market funds, at carrying value	13.6	0
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock of subsidiaries	165.2
Level 1 [Member] | IPL [Member]
Assets:
Money market funds, at carrying value	1.1	0
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock	154.8
Level 1 [Member] | WPL [Member]
Assets:
Money market funds, at carrying value	12.5	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock	10.4
Level 1 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivatives	0	0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivatives	0	0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivatives	0	0
Level 2 [Member] | Alliant Energy Corporation [Member]
Assets:
Money market funds, at carrying value	0	0
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	3,598.2
Cumulative preferred stock of subsidiaries	50.4
Level 2 [Member] | IPL [Member]
Assets:
Money market funds, at carrying value	0	0
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,696.1
Cumulative preferred stock	0
Level 2 [Member] | WPL [Member]
Assets:
Money market funds, at carrying value	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,492.4
Cumulative preferred stock	50.4
Level 2 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member]
Assets:
Derivative assets	11.9	3.4
Capitalization and liabilities:
Derivatives	40.3	64.8
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	8.1	1.3
Capitalization and liabilities:
Derivatives	18.3	28.6
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	3.8	2.1
Capitalization and liabilities:
Derivatives	22.0	36.2
Level 3 [Member] | Alliant Energy Corporation [Member]
Assets:
Money market funds, at carrying value	0	0
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	0.5
Cumulative preferred stock of subsidiaries	0
Level 3 [Member] | IPL [Member]
Assets:
Money market funds, at carrying value	0	0
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock	0
Level 3 [Member] | WPL [Member]
Assets:
Money market funds, at carrying value	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock	0
Level 3 [Member] | Commodity Contracts [Member] | Alliant Energy Corporation [Member]
Assets:
Derivative assets	34.6	12.3
Capitalization and liabilities:
Derivatives	17.0	13.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	20.0	9.3
Capitalization and liabilities:
Derivatives	4.5	5.0
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	14.6	3.0
Capitalization and liabilities:
Derivatives	$ 12.5	$ 8.2