Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets:
Deferred proceeds	$ 187.7	$ 119.4
Liabilities and equity:
Long-term debt (including current maturities)	6,919.9	5,860.8
Commodity Contracts [Member]
Assets:
Derivatives	26.8	28.3
Liabilities and equity:
Derivatives	37.6	23.3
Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
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 2 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and equity:
Long-term debt (including current maturities)	6,917.9	5,858.4
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.8	8.9
Liabilities and equity:
Derivatives	36.8	16.1
Level 3 [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	2.0	2.4
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	22.0	19.4
Liabilities and equity:
Derivatives	0.8	7.2
Carrying Amount [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	6,190.2	5,502.8
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	26.8	28.3
Liabilities and equity:
Derivatives	37.6	23.3
IPL [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	3,489.1	2,691.2
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	22.3	17.7
Liabilities and equity:
Derivatives	17.4	11.2
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 [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)	3,489.1	2,691.2
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.8	4.0
Liabilities and equity:
Derivatives	16.6	6.5
IPL [Member] | Level 3 [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	19.5	13.7
Liabilities and equity:
Derivatives	0.8	4.7
IPL [Member] | Carrying Amount [Member]
Assets:
Deferred proceeds	187.7	119.4
Liabilities and equity:
Long-term debt (including current maturities)	3,147.3	2,552.3
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	22.3	17.7
Liabilities and equity:
Derivatives	17.4	11.2
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,268.2	2,043.7
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.5	10.6
Liabilities and equity:
Derivatives	20.2	12.1
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)	2,268.2	2,043.7
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.0	4.9
Liabilities and equity:
Derivatives	20.2	9.6
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	2.5	5.7
Liabilities and equity:
Derivatives	0.0	2.5
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,932.7	1,834.9
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.5	10.6
Liabilities and equity:
Derivatives	$ 20.2	$ 12.1