Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,269.0	$ 1,201.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,269.0	1,201.5
Fair value of plan assets	950.7	895.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,303.1	1,244.3
Fair value of plan assets	950.7	895.7
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	222.3	220.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	222.3	220.1
Fair value of plan assets	111.1	105.8
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	107.5	102.8
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	573.1	546.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	573.1	546.7
Fair value of plan assets	443.7	422.0
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	592.9	570.4
Fair value of plan assets	443.7	422.0
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	89.4	90.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	89.4	90.1
Fair value of plan assets	72.9	68.2
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets		68.2
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	548.1	513.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	548.1	513.2
Fair value of plan assets	415.0	389.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	559.8	529.2
Fair value of plan assets	415.0	389.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	90.4	88.9
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	90.4	88.9
Fair value of plan assets	$ 18.7	$ 18.6