Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 32.0	$ 33.0
Capitalization and liabilities:
Derivative liabilities	44.0	32.0
IPL [Member]
Assets:
Derivative assets	29.3	24.7
Capitalization and liabilities:
Derivative liabilities	19.3	16.2
WPL [Member]
Assets:
Derivative assets	2.7	8.3
Capitalization and liabilities:
Derivative liabilities	24.7	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	198.9	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	198.9	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)	4,210.7	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)	1,937.2	2,053.0
Cumulative preferred stock	0.0	0.0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,824.0	1,908.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	3.0	2.6
Capitalization and liabilities:
Derivative liabilities	15.6	19.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	1.9	2.4
Capitalization and liabilities:
Derivative liabilities	10.2	13.3
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	1.1	0.2
Capitalization and liabilities:
Derivative liabilities	5.4	6.2
Level 3 [Member]
Assets:
Deferred proceeds	73.4	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	73.4	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	32.0	36.0
Capitalization and liabilities:
Derivative liabilities	31.4	18.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	28.8	25.6
Capitalization and liabilities:
Derivative liabilities	10.5	6.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	3.2	10.4
Capitalization and liabilities:
Derivative liabilities	20.9	11.9
Fair Value [Member]
Assets:
Deferred proceeds	73.4	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	4,214.4	4,418.2
Cumulative preferred stock	198.9	200.2
Fair Value [Member] | IPL [Member]
Assets:
Deferred proceeds	73.4	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	1,937.2	2,053.0
Cumulative preferred stock	198.9	200.2
Fair Value [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,824.0	1,908.9
Fair Value [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	35.0	38.6
Capitalization and liabilities:
Derivative liabilities	47.0	37.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	30.7	28.0
Capitalization and liabilities:
Derivative liabilities	20.7	19.5
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	4.3	10.6
Capitalization and liabilities:
Derivative liabilities	$ 26.3	$ 18.1