Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Derivative assets	$ 32.3	$ 23.5
Deferred proceeds	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,819.8	3,712.3
Cumulative preferred stock	183.8	167.0
Derivative liabilities	4.9	17.6
IPL [Member]
Assets:
Derivative assets	9.7	19.5
Deferred proceeds	170.8	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,779.6	1,726.4
Cumulative preferred stock	183.8	167.0
Derivative liabilities	0.9	3.6
WPL [Member]
Assets:
Derivative assets	22.6	4.0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,582.5	1,532.9
Derivative liabilities	4.0	14.0
Commodity Contracts [Member]
Assets:
Derivative assets	41.3	26.7
Capitalization and liabilities:
Derivative liabilities	9.0	20.8
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	15.9	21.1
Capitalization and liabilities:
Derivative liabilities	2.2	5.2
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	25.4	5.6
Capitalization and liabilities:
Derivative liabilities	6.8	15.6
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	183.8	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	183.8	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)	3,816.5	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)	1,779.6	1,726.4
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,582.5	1,532.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	11.2	4.7
Capitalization and liabilities:
Derivative liabilities	2.8	3.2
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	6.8	3.0
Capitalization and liabilities:
Derivative liabilities	0.9	1.7
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	4.4	1.7
Capitalization and liabilities:
Derivative liabilities	1.9	1.5
Level 3 [Member]
Assets:
Deferred proceeds	170.8	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	170.8	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	30.1	22.0
Capitalization and liabilities:
Derivative liabilities	6.2	17.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	9.1	18.1
Capitalization and liabilities:
Derivative liabilities	1.3	3.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	21.0	3.9
Capitalization and liabilities:
Derivative liabilities	$ 4.9	$ 14.1