Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Assets:
Deferred proceeds	$ 239.7	$ 172.0
Liabilities and Equity:
Long-term debt (including current maturities)	4,871.6	4,336.1
Cumulative preferred stock of IPL	215.4	206.6
IPL [Member]
Assets:
Deferred proceeds	239.7	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	2,495.8	2,092.7
Cumulative preferred stock of IPL	215.4	206.6
WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,920.4	1,793.0
Commodity Contracts [Member]
Assets:
Derivatives	27.8	18.4
Liabilities and Equity:
Derivatives	36.9	64.6
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	22.2	15.5
Liabilities and Equity:
Derivatives	9.0	23.4
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	5.6	2.9
Liabilities and Equity:
Derivatives	27.9	41.2
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	215.4	206.6
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	215.4	206.6
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)	4,868.3	4,332.4
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)	2,495.8	2,092.7
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,920.4	1,793.0
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	1.9	2.5
Liabilities and Equity:
Derivatives	3.1	16.0
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	1.2	2.0
Liabilities and Equity:
Derivatives	1.4	8.0
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.7	0.5
Liabilities and Equity:
Derivatives	1.7	8.0
Level 3 [Member]
Assets:
Deferred proceeds	239.7	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	3.3	3.7
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	239.7	172.0
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	25.9	15.9
Liabilities and Equity:
Derivatives	33.8	48.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	21.0	13.5
Liabilities and Equity:
Derivatives	7.6	15.4
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	4.9	2.4
Liabilities and Equity:
Derivatives	26.2	33.2
Carrying Amount [Member]
Assets:
Deferred proceeds	239.7	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	4,130.9	3,835.6
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	239.7	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	2,153.1	1,856.9
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,534.9	1,533.9
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	27.8	18.4
Liabilities and Equity:
Derivatives	36.9	64.6
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	22.2	15.5
Liabilities and Equity:
Derivatives	9.0	23.4
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	5.6	2.9
Liabilities and Equity:
Derivatives	$ 27.9	$ 41.2