Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Derivative assets	$ 41.3	$ 26.7
Deferred proceeds (sales of receivables)	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,819.8	3,712.3
Cumulative preferred stock	200.0	200.0
Cumulative preferred stock	183.8	167.0
Derivative liabilities	9.0	20.8
IPL [Member]
Assets:
Derivative assets	15.9	21.1
Deferred proceeds (sales of receivables)	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,779.6	1,726.4
Cumulative preferred stock	200.0	200.0
Cumulative preferred stock	183.8	167.0
Derivative liabilities	2.2	5.2
WPL [Member]
Assets:
Derivative assets	25.4	5.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,582.5	1,532.9
Derivative liabilities	6.8	15.6
Carrying Amount [Member]
Assets:
Derivative assets	41.3	26.7
Deferred proceeds (sales of receivables)	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,339.4	3,336.3
Cumulative preferred stock	200.0	200.0
Derivative liabilities	9.0	20.8
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	15.9	21.1
Deferred proceeds (sales of receivables)	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,558.6	1,558.4
Cumulative preferred stock	200.0	200.0
Derivative liabilities	2.2	5.2
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	25.4	5.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,332.2	1,332.1
Derivative liabilities	6.8	15.6
Fair Value [Member]
Assets:
Derivative assets	41.3	26.7
Deferred proceeds (sales of receivables)	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,819.8	3,712.3
Cumulative preferred stock	183.8	167.0
Derivative liabilities	9.0	20.8
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	15.9	21.1
Deferred proceeds (sales of receivables)	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,779.6	1,726.4
Cumulative preferred stock	183.8	167.0
Derivative liabilities	2.2	5.2
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	25.4	5.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,582.5	1,532.9
Derivative liabilities	$ 6.8	$ 15.6