Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets:
Deferred proceeds	$ 172.0	$ 177.2
Liabilities and equity:
Long-term debt (including current maturities)	3,870.5	3,802.8
IPL [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	1,875.0	1,775.0
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,550.0	1,580.6
Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock of IPL	206.6	200.2
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock of IPL	206.6	200.2
Level 1 [Member] | WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and equity:
Derivatives	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and equity:
Derivatives	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and equity:
Derivatives	0.0	0.0
Level 2 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	4,332.4	4,414.9
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	2,092.7	2,053.0
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,793.0	1,908.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.5	2.6
Liabilities and equity:
Derivatives	16.0	19.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	2.0	2.4
Liabilities and equity:
Derivatives	8.0	13.3
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.5	0.2
Liabilities and equity:
Derivatives	8.0	6.2
Level 3 [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	3.7	3.3
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	15.9	36.0
Liabilities and equity:
Derivatives	48.6	18.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	13.5	25.6
Liabilities and equity:
Derivatives	15.4	6.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	2.4	10.4
Liabilities and equity:
Derivatives	33.2	11.9
Carrying Value [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	3,835.6	3,767.3
Cumulative preferred stock of IPL	200.0	200.0
Carrying Value [Member] | IPL [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	1,856.9	1,758.0
Cumulative preferred stock of IPL	200.0	200.0
Carrying Value [Member] | WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,533.9	1,563.1
Carrying Value [Member] | Commodity Contracts [Member]
Assets:
Derivatives	18.4	38.6
Liabilities and equity:
Derivatives	64.6	37.6
Carrying Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	15.5	28.0
Liabilities and equity:
Derivatives	23.4	19.5
Carrying Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	2.9	10.6
Liabilities and equity:
Derivatives	41.2	18.1
Fair Value [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	4,336.1	4,418.2
Cumulative preferred stock of IPL	206.6	200.2
Fair Value [Member] | IPL [Member]
Assets:
Deferred proceeds	172.0	177.2
Liabilities and equity:
Long-term debt (including current maturities)	2,092.7	2,053.0
Cumulative preferred stock of IPL	206.6	200.2
Fair Value [Member] | WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,793.0	1,908.9
Fair Value [Member] | Commodity Contracts [Member]
Assets:
Derivatives	18.4	38.6
Liabilities and equity:
Derivatives	64.6	37.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	15.5	28.0
Liabilities and equity:
Derivatives	23.4	19.5
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	2.9	10.6
Liabilities and equity:
Derivatives	$ 41.2	$ 18.1