Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets:
Derivative assets	$ 72.2	$ 23.5
Deferred proceeds	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	3,852.6	3,712.3
Cumulative preferred stock	200.2	167.0
Derivative liabilities	7.9	17.6
IPL [Member]
Assets:
Derivative assets	39.8	19.5
Deferred proceeds	160.3	203.5
Capitalization and liabilities:
Long-term debt (including current maturities)	1,778.2	1,726.4
Cumulative preferred stock	200.2	167.0
Derivative liabilities	3.6	3.6
WPL [Member]
Assets:
Derivative assets	32.4	4.0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,618.0	1,532.9
Derivative liabilities	4.3	14.0
Commodity Contracts [Member]
Assets:
Derivative assets	83.1	26.7
Capitalization and liabilities:
Derivative liabilities	14.1	20.8
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	44.7	21.1
Capitalization and liabilities:
Derivative liabilities	8.2	5.2
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	38.4	5.6
Capitalization and liabilities:
Derivative liabilities	5.9	15.6
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	200.2	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	200.2	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)	3,849.3	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)	1,778.2	1,726.4
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,618.0	1,532.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	6.3	4.7
Capitalization and liabilities:
Derivative liabilities	5.6	3.2
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	4.3	3.0
Capitalization and liabilities:
Derivative liabilities	4.3	1.7
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	2.0	1.7
Capitalization and liabilities:
Derivative liabilities	1.3	1.5
Level 3 [Member]
Assets:
Deferred proceeds	160.3	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	160.3	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	76.8	22.0
Capitalization and liabilities:
Derivative liabilities	8.5	17.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	40.4	18.1
Capitalization and liabilities:
Derivative liabilities	3.9	3.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	36.4	3.9
Capitalization and liabilities:
Derivative liabilities	$ 4.6	$ 14.1