Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 10.3	$ 33.0
Deferred proceeds	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	36.1	32.0
IPL [Member]
Assets:
Derivative assets	5.1	24.7
Deferred proceeds	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	18.6	16.2
WPL [Member]
Assets:
Derivative assets	5.2	8.3
Capitalization and liabilities:
Long-term debt (including current maturities)	1,938.9	1,908.9
Derivative liabilities	17.5	15.8
Commodity Contracts [Member]
Assets:
Derivative assets	16.9	38.6
Capitalization and liabilities:
Derivative liabilities	42.7	37.6
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	6.5	28.0
Capitalization and liabilities:
Derivative liabilities	20.0	19.5
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	10.4	10.6
Capitalization and liabilities:
Derivative liabilities	22.7	18.1
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	203.7	200.2
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	203.7	200.2
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)	4,487.4	4,414.9
Cumulative preferred stock	0	0
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	2,092.9	2,053.0
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,938.9	1,908.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	2.2	2.6
Capitalization and liabilities:
Derivative liabilities	14.9	19.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	1.7	2.4
Capitalization and liabilities:
Derivative liabilities	10.2	13.3
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	0.5	0.2
Capitalization and liabilities:
Derivative liabilities	4.7	6.2
Level 3 [Member]
Assets:
Deferred proceeds	117.7	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	3.0	3.3
Cumulative preferred stock	0	0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	117.7	177.2
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	14.7	36.0
Capitalization and liabilities:
Derivative liabilities	27.8	18.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	4.8	25.6
Capitalization and liabilities:
Derivative liabilities	9.8	6.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	9.9	10.4
Capitalization and liabilities:
Derivative liabilities	$ 18.0	$ 11.9