Debt (Long-term Debt) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 3,149.9		$ 2,716.3
Commercial paper	217.5		102.8
Current maturities	(1.5)		(1.4)
Unamortized debt (discount) and premium, net	(11.8)		(11.8)
Long-term debt, net	3,136.6		2,703.1
Remaining maturity	8 days
Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	1,275.0		1,275.0
Senior Debentures [Member] | 3.3% senior debenture due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	150.0		150.0
Interest rate	3.30%
Senior Debentures [Member] | 5.875% senior debenture due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	100.0		100.0
Interest rate	5.875%
Senior Debentures [Member] | 7.25% senior debenture due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	7.25%
Senior Debentures [Member] | 3.65% senior debenture due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	200.0		200.0
Interest rate	3.65%
Senior Debentures [Member] | 5.5% senior debenture due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	50.0		50.0
Interest rate	5.50%
Senior Debentures [Member] | 6.45% senior debenture due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	100.0		100.0
Interest rate	6.45%
Senior Debentures [Member] | 6.3% senior debenture due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	125.0		125.0
Interest rate	6.30%
Senior Debentures [Member] | 6.25% senior debenture due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	300.0		300.0
Interest rate	6.25%
Debentures [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	1,300.0		1,050.0
Debentures [Member] | 5% debenture due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	5.00%
Debentures [Member] | 4.6% debenture due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	150.0		150.0
Interest rate	4.60%
Debentures [Member] | 2.25% debenture due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0	[1]	0	[1]
Interest rate	2.25%
Debentures [Member] | 6.25% debenture due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	100.0		100.0
Interest rate	6.25%
Debentures [Member] | 6.375% debenture due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	300.0		300.0
Interest rate	6.375%
Debentures [Member] | 7.6% debenture due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	7.60%
Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	77.5		77.5
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	38.4		38.4
Interest rate	5.00%
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond due 2014 and 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	24.5		24.5
Interest rate	5.00%
Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	14.6		14.6
Interest rate	5.375%
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	497.4		313.8
Long-term debt [Member]
Debt Instrument [Line Items]
Commercial paper	50.0	[2]	0	[2]
Interest rate	0.40%
Senior Notes [Member] | 4% senior notes, due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	4.00%
Senior Notes [Member] | 3.45% senior notes, due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	75.0	[3]	0	[3]
Interest rate	3.45%
Term Loan Credit Agreement [Member] | Term loan credit agreement through 2014, 1.1% at December 31, 2012 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	60.0	[4]	0	[4]
Interest rate	1.10%
Senior Secured Notes [Member] | 5.06% senior secured notes, due 2013 to 2024 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	61.9		63.3
Interest rate	5.06%
Other [Member] | Other, 1% at December 31, 2012, due 2013 to 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	0.5		0.5
Interest rate	1.00%
IPL [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	1,363.4		1,313.4
Commercial paper	26.3		7.1
Current maturities	0		0
Unamortized debt (discount) and premium, net	(3.9)		(4.4)
Long-term debt, net	1,359.5		1,309.0
IPL [Member] | Senior Debentures [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	1,275.0		1,275.0
IPL [Member] | Senior Debentures [Member] | 3.3% senior debenture due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	150.0		150.0
Interest rate	3.30%
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	100.0		100.0
Interest rate	5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	7.25%
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	200.0		200.0
Interest rate	3.65%
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	50.0		50.0
Interest rate	5.50%
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	100.0		100.0
Interest rate	6.45%
IPL [Member] | Senior Debentures [Member] | 6.3% senior debenture due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	125.0		125.0
Interest rate	6.30%
IPL [Member] | Senior Debentures [Member] | 6.25% senior debenture due 2039 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	300.0		300.0
Interest rate	6.25%
IPL [Member] | Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	38.4		38.4
IPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	38.4		38.4
Interest rate	5.00%
IPL [Member] | Other Long Term Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	50.0		0
IPL [Member] | Long-term debt [Member]
Debt Instrument [Line Items]
Commercial paper	50.0	[2]	0	[2]
Interest rate	0.40%
WPL [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	1,339.1		1,089.1
Commercial paper	86.6		25.7
Current maturities	0		0
Unamortized debt (discount) and premium, net	(7.6)		(6.9)
Long-term debt, net	1,331.5		1,082.2
WPL [Member] | Debentures [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	1,300.0		1,050.0
WPL [Member] | Debentures [Member] | 5% debenture due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	5.00%
WPL [Member] | Debentures [Member] | 4.6% debenture due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	150.0		150.0
Interest rate	4.60%
WPL [Member] | Debentures [Member] | 2.25% debenture due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0	[1]	0	[1]
Interest rate	2.25%
WPL [Member] | Debentures [Member] | 6.25% debenture due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	100.0		100.0
Interest rate	6.25%
WPL [Member] | Debentures [Member] | 6.375% debenture due 2037 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	300.0		300.0
Interest rate	6.375%
WPL [Member] | Debentures [Member] | 7.6% debenture due 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	250.0		250.0
Interest rate	7.60%
WPL [Member] | Pollution Control Revenue Bonds [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	39.1		39.1
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond due 2014 and 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	24.5		24.5
Interest rate	5.00%
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 14.6		$ 14.6
Interest rate	5.375%

[1] In 2012, WPL issued $250.0 million of 2.25% debentures due 2022. The proceeds from the issuance were used by WPL to fund a portion of the purchase price of Riverside.

[2] As of December 31, 2012, $50.0 million of commercial paper was recorded in “Long-term debt, net” on Alliant Energy’s and IPL’s Consolidated Balance Sheets due to the existence of long-term credit facilities that back-stop this commercial paper balance, along with Alliant Energy’s and IPL’s intent and ability to refinance these balances on a long-term basis. As of December 31, 2012, this commercial paper balance had a remaining maturity of 8 days.

[3] In 2012, Corporate Services issued $75 million of 3.45% senior notes due 2022. The proceeds from the issuance were used by Corporate Services to repay short-term debt primarily incurred for the purchase of the corporate headquarters building and for general working capital purposes.

[4] In 2012, Franklin County Holdings LLC, Resources’ wholly-owned subsidiary, entered into a $60.0 million variable-rate term loan credit agreement that exists through 2014 to fund a portion of the costs of its Franklin County wind project, which was placed into service in the fourth quarter of 2012.