Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Assets:
Derivative assets	$ 23.5	$ 19.3
Deferred proceeds	203.5	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,712.3	3,860.5
Cumulative preferred stock	167.0
Cumulative preferred stock		212.6
Derivative liabilities	17.6	33.5
IPL [Member]
Assets:
Derivative assets	19.5	14.5
Deferred proceeds	203.5	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,726.4	1,679.9
Cumulative preferred stock	167.0	151.8
Derivative liabilities	3.6	13.1
WPL [Member]
Assets:
Derivative assets	4.0	4.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,532.9	1,713.3
Cumulative preferred stock	0	60.8
Derivative liabilities	14.0	20.4
Commodity Contracts [Member]
Assets:
Derivative assets	26.7	26.2
Capitalization and liabilities:
Derivative liabilities	20.8	40.4
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	21.1	17.5
Capitalization and liabilities:
Derivative liabilities	5.2	16.1
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	5.6	8.7
Capitalization and liabilities:
Derivative liabilities	15.6	24.3
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	167.0
Cumulative preferred stock		162.3
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	167.0	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)	3,711.8	3,860.0
Cumulative preferred stock	0
Cumulative preferred stock		50.3
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,726.4	1,679.9
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,532.9	1,713.3
Cumulative preferred stock	0	50.3
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	4.7	4.8
Capitalization and liabilities:
Derivative liabilities	3.2	30.9
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	3.0	3.1
Capitalization and liabilities:
Derivative liabilities	1.7	14.2
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	1.7	1.7
Capitalization and liabilities:
Derivative liabilities	1.5	16.7
Level 3 [Member]
Assets:
Deferred proceeds	203.5	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	0.5	0.5
Cumulative preferred stock	0
Cumulative preferred stock		0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	203.5	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	22.0	21.4
Capitalization and liabilities:
Derivative liabilities	17.6	9.5
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	18.1	14.4
Capitalization and liabilities:
Derivative liabilities	3.5	1.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	3.9	7.0
Capitalization and liabilities:
Derivative liabilities	$ 14.1	$ 7.6