Benefit Plans (Accumulated Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,071.7	$ 1,155.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	406.5	1,155.5
Fair value of plan assets	347.6	965.6
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,113.4	1,207.5
Fair value of plan assets	1,022.9	965.6
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	208.7	223.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	208.7	223.2
Fair value of plan assets	124.9	123.1
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	124.9	123.1
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	491.5	530.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	159.3	530.4
Fair value of plan assets	144.6	458.8
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	514.0	559.2
Fair value of plan assets	485.9	458.8
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	87.8	96.0
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	87.8	96.0
Fair value of plan assets	81.2	78.8
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	81.2	78.8
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	446.7	490.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	115.6	490.2
Fair value of plan assets	106.8	415.4
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	460.8	506.7
Fair value of plan assets	438.8	415.4
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	85.6	89.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	85.6	89.1
Fair value of plan assets	21.7	22.3
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	$ 21.7	$ 22.3