Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 18.0	$ 33.0
Capitalization and liabilities:
Derivative liabilities	56.8	32.0
IPL [Member]
Assets:
Derivative assets	17.3	24.7
Capitalization and liabilities:
Derivative liabilities	20.8	16.2
WPL [Member]
Assets:
Derivative assets	0.7	8.3
Capitalization and liabilities:
Derivative liabilities	36.0	15.8
Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
Capitalization and liabilities:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock	201.3	200.2
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Capitalization and liabilities:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock	201.3	200.2
Level 1 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	0.0	0.0
Capitalization and liabilities:
Derivative liabilities	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	0.0	0.0
Capitalization and liabilities:
Derivative liabilities	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	0.0	0.0
Capitalization and liabilities:
Derivative liabilities	0.0	0.0
Level 2 [Member]
Assets:
Deferred proceeds	0.0	0.0
Capitalization and liabilities:
Long-term debt (including current maturities)	4,390.6	4,414.9
Cumulative preferred stock	0.0	0.0
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Capitalization and liabilities:
Long-term debt (including current maturities)	2,115.3	2,053.0
Cumulative preferred stock	0.0	0.0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,825.5	1,908.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	2.6	2.6
Capitalization and liabilities:
Derivative liabilities	16.8	19.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	2.1	2.4
Capitalization and liabilities:
Derivative liabilities	9.4	13.3
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	0.5	0.2
Capitalization and liabilities:
Derivative liabilities	7.4	6.2
Level 3 [Member]
Assets:
Deferred proceeds	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	3.7	3.3
Cumulative preferred stock	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock	0.0	0.0
Level 3 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	17.7	36.0
Capitalization and liabilities:
Derivative liabilities	42.3	18.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	16.4	25.6
Capitalization and liabilities:
Derivative liabilities	12.6	6.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	1.3	10.4
Capitalization and liabilities:
Derivative liabilities	29.7	11.9
Fair Value [Member]
Assets:
Deferred proceeds	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	4,394.3	4,418.2
Cumulative preferred stock	201.3	200.2
Fair Value [Member] | IPL [Member]
Assets:
Deferred proceeds	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	2,115.3	2,053.0
Cumulative preferred stock	201.3	200.2
Fair Value [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,825.5	1,908.9
Fair Value [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	20.3	38.6
Capitalization and liabilities:
Derivative liabilities	59.1	37.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	18.5	28.0
Capitalization and liabilities:
Derivative liabilities	22.0	19.5
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	1.8	10.6
Capitalization and liabilities:
Derivative liabilities	$ 37.1	$ 18.1