Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,139.9	$ 1,269.0
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,139.9	1,269.0
Fair value of plan assets	808.6	950.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,175.0	1,303.1
Fair value of plan assets	808.6	950.7
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	206.1	222.3
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	206.1	222.3
Fair value of plan assets	99.1	111.1
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	76.8	107.5
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	514.3	573.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	514.3	573.1
Fair value of plan assets	377.8	443.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	533.9	592.9
Fair value of plan assets	377.8	443.7
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	82.5	89.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	82.5	89.4
Fair value of plan assets	66.7	72.9
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	47.7	72.9
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	494.8	548.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	494.8	548.1
Fair value of plan assets	352.2	415.0
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	506.6	559.8
Fair value of plan assets	352.2	415.0
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	83.2	90.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	83.2	90.4
Fair value of plan assets	$ 16.7	$ 18.7