Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets:
Deferred proceeds	$ 211.1	$ 172.0
Liabilities and equity:
Long-term debt (including current maturities)	4,799.0	4,336.1
IPL's cumulative preferred stock	194.8	206.6
Commodity Contracts [Member]
Assets:
Derivatives	41.4	18.4
Liabilities and equity:
Derivatives	28.6	64.6
Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
IPL's cumulative preferred stock	194.8	206.6
Level 1 [Member] | Commodity Contracts [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)	4,795.7	4,332.4
IPL's cumulative preferred stock	0.0	0.0
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.6	2.5
Liabilities and equity:
Derivatives	0.5	16.0
Level 3 [Member]
Assets:
Deferred proceeds	211.1	172.0
Liabilities and equity:
Long-term debt (including current maturities)	3.3	3.7
IPL's cumulative preferred stock	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	36.8	15.9
Liabilities and equity:
Derivatives	28.1	48.6
Carrying Amount [Member]
Assets:
Deferred proceeds	211.1	172.0
Liabilities and equity:
Long-term debt (including current maturities)	4,320.2	3,835.6
IPL's cumulative preferred stock	200.0	200.0
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	41.4	18.4
Liabilities and equity:
Derivatives	28.6	64.6
IPL [Member]
Assets:
Deferred proceeds	211.1	172.0
Liabilities and equity:
Long-term debt (including current maturities)	2,352.3	2,092.7
IPL's cumulative preferred stock	194.8	206.6
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	20.8	15.5
Liabilities and equity:
Derivatives	8.3	23.4
IPL [Member] | Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
IPL's cumulative preferred stock	194.8	206.6
IPL [Member] | Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and equity:
Derivatives	0.0	0.0
IPL [Member] | Level 2 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	2,352.3	2,092.7
IPL's cumulative preferred stock	0.0	0.0
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.8	2.0
Liabilities and equity:
Derivatives	0.4	8.0
IPL [Member] | Level 3 [Member]
Assets:
Deferred proceeds	211.1	172.0
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
IPL's cumulative preferred stock	0.0	0.0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	18.0	13.5
Liabilities and equity:
Derivatives	7.9	15.4
IPL [Member] | Carrying Amount [Member]
Assets:
Deferred proceeds	211.1	172.0
Liabilities and equity:
Long-term debt (including current maturities)	2,153.5	1,856.9
IPL's cumulative preferred stock	200.0	200.0
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	20.8	15.5
Liabilities and equity:
Derivatives	8.3	23.4
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,807.4	1,793.0
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	20.6	2.9
Liabilities and equity:
Derivatives	20.3	41.2
WPL [Member] | Level 1 [Member]
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
WPL [Member] | Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and equity:
Derivatives	0.0	0.0
WPL [Member] | Level 2 [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,807.4	1,793.0
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	1.8	0.5
Liabilities and equity:
Derivatives	0.1	8.0
WPL [Member] | Level 3 [Member]
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
WPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	18.8	2.4
Liabilities and equity:
Derivatives	20.2	33.2
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,535.2	1,533.9
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	20.6	2.9
Liabilities and equity:
Derivatives	$ 20.3	$ 41.2