Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets:
Derivative assets	$ 32.3	$ 19.3
Deferred proceeds	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,553.6	3,860.5
Cumulative preferred stock		212.6
Cumulative preferred stock	169.5
Derivative liabilities	23.5	33.0
IPL [Member]
Assets:
Derivative assets	29.2	14.5
Deferred proceeds	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,553.8	1,679.9
Cumulative preferred stock	169.5	151.8
Derivative liabilities	5.3	12.6
WPL [Member]
Assets:
Derivative assets	3.1	4.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,541.9	1,713.3
Cumulative preferred stock	0	60.8
Derivative liabilities	18.2	20.4
Commodity Contracts [Member]
Assets:
Derivative assets	36.8	26.2
Capitalization and liabilities:
Derivative liabilities	27.1	40.4
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	31.5	17.5
Capitalization and liabilities:
Derivative liabilities	6.7	16.1
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	5.3	8.7
Capitalization and liabilities:
Derivative liabilities	20.4	24.3
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock		162.3
Cumulative preferred stock	169.5
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	169.5	151.8
Level 1 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	0	10.5
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)	3,553.2	3,860.0
Cumulative preferred stock		50.3
Cumulative preferred stock	0
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,553.8	1,679.9
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,541.9	1,713.3
Cumulative preferred stock	0	50.3
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	3.2	4.8
Capitalization and liabilities:
Derivative liabilities	10.7	30.9
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	1.9	3.1
Capitalization and liabilities:
Derivative liabilities	5.7	14.2
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	1.3	1.7
Capitalization and liabilities:
Derivative liabilities	5.0	16.7
Level 3 [Member]
Assets:
Deferred proceeds	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	0.4	0.5
Cumulative preferred stock		0
Cumulative preferred stock	0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	86.9	66.8
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
Cumulative preferred stock	0	0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	33.6	21.4
Capitalization and liabilities:
Derivative liabilities	16.4	9.5
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	29.6	14.4
Capitalization and liabilities:
Derivative liabilities	1.0	1.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	4.0	7.0
Capitalization and liabilities:
Derivative liabilities	$ 15.4	$ 7.6