Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,201.5	$ 1,166.0
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,201.5	1,166.0
Fair value of plan assets	895.7	895.0
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,244.3	1,206.3
Fair value of plan assets	895.7	895.0
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	220.1	221.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	220.1	221.4
Fair value of plan assets	105.8	106.9
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	102.8	101.4
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	546.7	531.0
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	546.7	531.0
Fair value of plan assets	422.0	422.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	570.4	556.1
Fair value of plan assets	422.0	422.7
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	90.1	91.3
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	90.1	91.3
Fair value of plan assets	68.2	69.2
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	68.2	68.6
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	513.2	493.8
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	513.2	493.8
Fair value of plan assets	389.7	386.8
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	529.2	505.9
Fair value of plan assets	389.7	386.8
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	88.9	89.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	88.9	89.7
Fair value of plan assets	$ 18.6	$ 18.7