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Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Assets:    
Derivative assets $ 60.5 $ 26.2
Deferred proceeds (sales of receivables) 69.3 66.8
Capitalization and liabilities:    
Long-term debt (including current maturities) 3,619.5 3,860.5
Cumulative preferred stock 200.0 145.1
Cumulative preferred stock 188.4  
Cumulative preferred stock 0 60.0
Cumulative preferred stock   212.6
Derivative liabilities 37.1 40.4
IPL [Member]
    
Assets:    
Derivative assets 45.1 17.5
Deferred proceeds (sales of receivables) 69.3 66.8
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,586.8 1,679.9
Cumulative preferred stock 200.0 145.1
Cumulative preferred stock 188.4 151.8
Derivative liabilities 9.7 16.1
WPL [Member]
    
Assets:    
Derivative assets 15.4 8.7
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,572.1 1,713.3
Cumulative preferred stock 0 60.0
Cumulative preferred stock 0 60.8
Derivative liabilities 27.4 24.3
Carrying Amount [Member]
    
Assets:    
Derivative assets 60.5 26.2
Deferred proceeds (sales of receivables) 69.3 66.8
Capitalization and liabilities:    
Long-term debt (including current maturities) 3,142.9 3,138.1
Cumulative preferred stock 200.0  
Cumulative preferred stock   205.1
Derivative liabilities 37.1 40.4
Carrying Amount [Member] | IPL [Member]
    
Assets:    
Derivative assets 45.1 17.5
Deferred proceeds (sales of receivables) 69.3 66.8
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,364.7 1,359.5
Cumulative preferred stock 200.0 145.1
Derivative liabilities 9.7 16.1
Carrying Amount [Member] | WPL [Member]
    
Assets:    
Derivative assets 15.4 8.7
Deferred proceeds (sales of receivables) 0 0
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,331.9 1,331.5
Cumulative preferred stock 0 60.0
Derivative liabilities 27.4 24.3
Fair Value [Member]
    
Assets:    
Derivative assets 60.5 26.2
Deferred proceeds (sales of receivables) 69.3 66.8
Capitalization and liabilities:    
Long-term debt (including current maturities) 3,619.5 3,860.5
Cumulative preferred stock 188.4  
Cumulative preferred stock   212.6
Derivative liabilities 37.1 40.4
Fair Value [Member] | IPL [Member]
    
Assets:    
Derivative assets 45.1 17.5
Deferred proceeds (sales of receivables) 69.3 66.8
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,586.8 1,679.9
Cumulative preferred stock 188.4 151.8
Derivative liabilities 9.7 16.1
Fair Value [Member] | WPL [Member]
    
Assets:    
Derivative assets 15.4 8.7
Deferred proceeds (sales of receivables) 0 0
Capitalization and liabilities:    
Long-term debt (including current maturities) 1,572.1 1,713.3
Cumulative preferred stock 0 60.8
Derivative liabilities $ 27.4 $ 24.3