Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,234.4	$ 1,139.9
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,234.4	1,139.9
Fair value of plan assets	930.4	808.6
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,279.7	1,175.0
Fair value of plan assets	930.4	808.6
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	213.6	206.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	213.6	206.1
Fair value of plan assets	104.9	99.1
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	557.8	514.3
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	557.8	514.3
Fair value of plan assets	436.0	377.8
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	581.2	533.9
Fair value of plan assets	436.0	377.8
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	85.7	82.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	85.7	82.5
Fair value of plan assets	71.9	66.7
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	535.9	494.8
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	535.9	494.8
Fair value of plan assets	406.0	352.2
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	550.1	506.6
Fair value of plan assets	406.0	352.2
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	85.3	83.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	85.3	83.2
Fair value of plan assets	$ 17.1	$ 16.7