Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Assets:
Derivative assets	$ 103.9	$ 23.5
Deferred proceeds	193.7	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,907.4	3,712.3
Cumulative preferred stock	199.2	167.0
Derivative liabilities	6.6	17.6
IPL [Member]
Assets:
Derivative assets	68.8	19.5
Deferred proceeds	193.7	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,821.3	1,726.4
Cumulative preferred stock	199.2	167.0
Derivative liabilities	2.4	3.6
WPL [Member]
Assets:
Derivative assets	35.1	4.0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,627.1	1,532.9
Derivative liabilities	4.2	14.0
Commodity Contracts [Member]
Assets:
Derivative assets	119.0	26.7
Capitalization and liabilities:
Derivative liabilities	13.3	20.8
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	73.0	21.1
Capitalization and liabilities:
Derivative liabilities	6.3	5.2
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	46.0	5.6
Capitalization and liabilities:
Derivative liabilities	7.0	15.6
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	199.2	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	199.2	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)	3,904.1	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)	1,821.3	1,726.4
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,627.1	1,532.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	10.6	4.7
Capitalization and liabilities:
Derivative liabilities	6.1	3.2
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	6.7	3.0
Capitalization and liabilities:
Derivative liabilities	4.2	1.7
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	3.9	1.7
Capitalization and liabilities:
Derivative liabilities	1.9	1.5
Level 3 [Member]
Assets:
Deferred proceeds	193.7	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	193.7	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	108.4	22.0
Capitalization and liabilities:
Derivative liabilities	7.2	17.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	66.3	18.1
Capitalization and liabilities:
Derivative liabilities	2.1	3.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	42.1	3.9
Capitalization and liabilities:
Derivative liabilities	$ 5.1	$ 14.1