Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory assets	$ 1,843.0	$ 1,844.7
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	842.5	817.6
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	501.2	524.0
Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	122.9	134.0
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	114.0	111.8
IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	109.4	108.2
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	47.0	39.5
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	19.8	21.1
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	86.2	88.5
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,404.9	1,399.3
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	797.4	776.8
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	251.3	262.5
IPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	81.8	87.9
IPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	78.2	76.2
IPL [Member] | IPL's DAEC PPA amendment [Member]
Regulatory Assets [Line Items]
Regulatory assets	109.4	108.2
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	20.5	18.3
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	19.8	21.1
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	46.5	48.3
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	438.1	445.4
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	45.1	40.8
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	249.9	261.5
WPL [Member] | Assets retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	41.1	46.1
WPL [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.8	35.6
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.5	21.2
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 39.7	$ 40.2