Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
Derivative assets	$ 33.0us-gaap_DerivativeAssets	$ 23.5us-gaap_DerivativeAssets
Deferred proceeds	177.2	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	4,418.2us-gaap_DebtInstrumentFairValue	3,712.3us-gaap_DebtInstrumentFairValue
Cumulative preferred stock	200.2lnt_PreferredStockFairValue	167.0lnt_PreferredStockFairValue
Derivative liabilities	32.0us-gaap_DerivativeLiabilities	17.6us-gaap_DerivativeLiabilities
IPL [Member]
Assets:
Derivative assets	24.7	19.5
Deferred proceeds	177.2	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	2,053.0	1,726.4
Cumulative preferred stock	200.2	167.0
Derivative liabilities	16.2	3.6
WPL [Member]
Assets:
Derivative assets	8.3	4.0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,908.9	1,532.9
Derivative liabilities	15.8	14.0
Commodity Contracts [Member]
Assets:
Derivative assets	38.6	26.7
Capitalization and liabilities:
Derivative liabilities	37.6	20.8
Commodity Contracts [Member] \| IPL [Member]
Assets:
Derivative assets	28.0	21.1
Capitalization and liabilities:
Derivative liabilities	19.5	5.2
Commodity Contracts [Member] \| WPL [Member]
Assets:
Derivative assets	10.6	5.6
Capitalization and liabilities:
Derivative liabilities	18.1	15.6
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	200.2	167.0
Level 1 [Member] \| IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	200.2	167.0
Level 1 [Member] \| WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Level 1 [Member] \| Commodity Contracts [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivative liabilities	0	0
Level 1 [Member] \| Commodity Contracts [Member] \| IPL [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivative liabilities	0	0
Level 1 [Member] \| Commodity Contracts [Member] \| WPL [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivative liabilities	0	0
Level 2 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	4,414.9	3,711.8
Cumulative preferred stock	0	0
Level 2 [Member] \| IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	2,053.0	1,726.4
Cumulative preferred stock	0	0
Level 2 [Member] \| WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,908.9	1,532.9
Level 2 [Member] \| Commodity Contracts [Member]
Assets:
Derivative assets	2.6	4.7
Capitalization and liabilities:
Derivative liabilities	19.5	3.2
Level 2 [Member] \| Commodity Contracts [Member] \| IPL [Member]
Assets:
Derivative assets	2.4	3.0
Capitalization and liabilities:
Derivative liabilities	13.3	1.7
Level 2 [Member] \| Commodity Contracts [Member] \| WPL [Member]
Assets:
Derivative assets	0.2	1.7
Capitalization and liabilities:
Derivative liabilities	6.2	1.5
Level 3 [Member]
Assets:
Deferred proceeds	177.2	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3.3	0.5
Cumulative preferred stock	0	0
Level 3 [Member] \| IPL [Member]
Assets:
Deferred proceeds	177.2	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	0	0
Level 3 [Member] \| WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Level 3 [Member] \| Commodity Contracts [Member]
Assets:
Derivative assets	36.0	22.0
Capitalization and liabilities:
Derivative liabilities	18.1	17.6
Level 3 [Member] \| Commodity Contracts [Member] \| IPL [Member]
Assets:
Derivative assets	25.6	18.1
Capitalization and liabilities:
Derivative liabilities	6.2	3.5
Level 3 [Member] \| Commodity Contracts [Member] \| WPL [Member]
Assets:
Derivative assets	10.4	3.9
Capitalization and liabilities:
Derivative liabilities	$ 11.9	$ 14.1

Assets: