Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets:
Derivative assets	$ 46.3	$ 19.3
Deferred proceeds	69.3	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,619.5	3,860.5
Cumulative preferred stock		212.6
Cumulative preferred stock	188.4
Derivative liabilities	22.9	33.0
IPL [Member]
Assets:
Derivative assets	43.4	14.5
Deferred proceeds	69.3	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,586.8	1,679.9
Cumulative preferred stock	188.4	151.8
Derivative liabilities	8.0	12.6
WPL [Member]
Assets:
Derivative assets	2.9	4.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,572.1	1,713.3
Cumulative preferred stock	0	60.8
Derivative liabilities	14.9	20.4
Commodity Contracts [Member]
Assets:
Derivative assets	60.5	26.2
Capitalization and liabilities:
Derivative liabilities	37.1	40.4
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	45.1	17.5
Capitalization and liabilities:
Derivative liabilities	9.7	16.1
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	15.4	8.7
Capitalization and liabilities:
Derivative liabilities	27.4	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	188.4
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	188.4	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)	3,619.1	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)	1,586.8	1,679.9
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,572.1	1,713.3
Cumulative preferred stock	0	50.3
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	13.7	4.8
Capitalization and liabilities:
Derivative liabilities	32.8	30.9
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	3.0	3.1
Capitalization and liabilities:
Derivative liabilities	8.2	14.2
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	10.7	1.7
Capitalization and liabilities:
Derivative liabilities	24.6	16.7
Level 3 [Member]
Assets:
Deferred proceeds	69.3	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	69.3	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	46.8	21.4
Capitalization and liabilities:
Derivative liabilities	4.3	9.5
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	42.1	14.4
Capitalization and liabilities:
Derivative liabilities	1.5	1.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	4.7	7.0
Capitalization and liabilities:
Derivative liabilities	$ 2.8	$ 7.6