Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 16.9	$ 38.6
Deferred proceeds (sales of receivables)	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	4,490.4us-gaap_DebtInstrumentFairValue	4,418.2us-gaap_DebtInstrumentFairValue
Cumulative preferred stock	203.7lnt_PreferredStockFairValue	200.2lnt_PreferredStockFairValue
Derivative liabilities	42.7	37.6
IPL [Member]
Assets:
Derivative assets	6.5	28.0
Deferred proceeds (sales of receivables)	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	2,092.9	2,053.0
Cumulative preferred stock	203.7	200.2
Derivative liabilities	20.0	19.5
WPL [Member]
Assets:
Derivative assets	10.4	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,938.9	1,908.9
Derivative liabilities	22.7	18.1
Carrying Value [Member]
Assets:
Derivative assets	16.9	38.6
Deferred proceeds (sales of receivables)	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	3,789.7	3,789.7
Cumulative preferred stock	200.0	200.0
Derivative liabilities	42.7	37.6
Carrying Value [Member] \| IPL [Member]
Assets:
Derivative assets	6.5	28.0
Deferred proceeds (sales of receivables)	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	1,768.8	1,768.7
Cumulative preferred stock	200.0	200.0
Derivative liabilities	20.0	19.5
Carrying Value [Member] \| WPL [Member]
Assets:
Derivative assets	10.4	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,574.0	1,573.9
Derivative liabilities	22.7	18.1
Fair Value [Member]
Assets:
Derivative assets	16.9	38.6
Deferred proceeds (sales of receivables)	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	4,490.4	4,418.2
Cumulative preferred stock	203.7	200.2
Derivative liabilities	42.7	37.6
Fair Value [Member] \| IPL [Member]
Assets:
Derivative assets	6.5	28.0
Deferred proceeds (sales of receivables)	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	2,092.9	2,053.0
Cumulative preferred stock	203.7	200.2
Derivative liabilities	20.0	19.5
Fair Value [Member] \| WPL [Member]
Assets:
Derivative assets	10.4	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,938.9	1,908.9
Derivative liabilities	$ 22.7	$ 18.1

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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