Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets:
Derivative assets	$ 83.1	$ 26.7
Deferred proceeds (sales of receivables)	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,852.6	3,712.3
Cumulative preferred stock	200.0	200.0
Cumulative preferred stock	200.2	167.0
Derivative liabilities	14.1	20.8
IPL [Member]
Assets:
Derivative assets	44.7	21.1
Deferred proceeds (sales of receivables)	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,778.2	1,726.4
Cumulative preferred stock	200.0	200.0
Cumulative preferred stock	200.2	167.0
Derivative liabilities	8.2	5.2
WPL [Member]
Assets:
Derivative assets	38.4	5.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,618.0	1,532.9
Derivative liabilities	5.9	15.6
Carrying Amount [Member]
Assets:
Derivative assets	83.1	26.7
Deferred proceeds (sales of receivables)	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,292.3	3,336.3
Cumulative preferred stock	200.0	200.0
Derivative liabilities	14.1	20.8
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	44.7	21.1
Deferred proceeds (sales of receivables)	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,520.3	1,558.4
Cumulative preferred stock	200.0	200.0
Derivative liabilities	8.2	5.2
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	38.4	5.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,324.0	1,332.1
Derivative liabilities	5.9	15.6
Fair Value [Member]
Assets:
Derivative assets	83.1	26.7
Deferred proceeds (sales of receivables)	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,852.6	3,712.3
Cumulative preferred stock	200.2	167.0
Derivative liabilities	14.1	20.8
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	44.7	21.1
Deferred proceeds (sales of receivables)	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,778.2	1,726.4
Cumulative preferred stock	200.2	167.0
Derivative liabilities	8.2	5.2
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	38.4	5.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,618.0	1,532.9
Derivative liabilities	$ 5.9	$ 15.6