v2.4.0.6
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.