Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets:
Deferred proceeds	$ 119.4	$ 222.1
Liabilities and equity:
Long-term debt (including current maturities)	5,860.8	5,447.5
Commodity Contracts [Member]
Assets:
Derivatives	28.3	25.1
Liabilities and equity:
Derivatives	23.3	41.7
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)	5,858.4	5,444.6
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	8.9	4.1
Liabilities and equity:
Derivatives	16.1	8.5
Level 3 [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	2.4	2.9
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	19.4	21.0
Liabilities and equity:
Derivatives	7.2	33.2
Carrying Amount [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	5,502.8	4,866.3
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	28.3	25.1
Liabilities and equity:
Derivatives	23.3	41.7
IPL [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	2,691.2	2,665.7
IPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	17.7	17.1
Liabilities and equity:
Derivatives	11.2	19.4
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 [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)	2,691.2	2,665.7
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.0	2.0
Liabilities and equity:
Derivatives	6.5	2.9
IPL [Member] | Level 3 [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	0.0	0.0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	13.7	15.1
Liabilities and equity:
Derivatives	4.7	16.5
IPL [Member] | Carrying Amount [Member]
Assets:
Deferred proceeds	119.4	222.1
Liabilities and equity:
Long-term debt (including current maturities)	2,552.3	2,406.0
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	17.7	17.1
Liabilities and equity:
Derivatives	11.2	19.4
WPL [Member]
Liabilities and equity:
Long-term debt (including current maturities)	2,043.7	2,147.9
WPL [Member] | Commodity Contracts [Member]
Assets:
Derivatives	10.6	8.0
Liabilities and equity:
Derivatives	12.1	22.3
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)	2,043.7	2,147.9
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.9	2.1
Liabilities and equity:
Derivatives	9.6	5.6
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	5.7	5.9
Liabilities and equity:
Derivatives	2.5	16.7
WPL [Member] | Carrying Amount [Member]
Liabilities and equity:
Long-term debt (including current maturities)	1,834.9	1,833.4
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	10.6	8.0
Liabilities and equity:
Derivatives	$ 12.1	$ 22.3