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Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Defined benefit pension plans [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 4.0 $ 3.4 $ 7.9 $ 6.7
Interest cost 12.2 12.9 24.5 25.9
Expected return on plan assets (18.5) (17.2) (37.0) (34.4)
Amortization of prior service cost (credit) 0 0 0.1 0.1
Amortization of actuarial loss 9.1 8.3 18.1 16.6
Total 6.8 7.4 13.6 14.9
Other Postretirement Benefits Plans [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1.6 1.8 3.2 3.5
Interest cost 2.1 2.5 4.2 5.1
Expected return on plan assets (2.0) (1.9) (4.0) (3.8)
Amortization of prior service cost (credit) (2.9) (3.0) (5.9) (6.0)
Amortization of actuarial loss 1.2 1.5 2.4 3.1
Total 0 0.9 (0.1) 1.9
IPL [Member] | Defined benefit pension plans [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 2.1 1.9 4.3 3.8
Interest cost 5.7 5.9 11.4 12.0
Expected return on plan assets (8.8) (8.1) (17.6) (16.3)
Amortization of prior service cost (credit) 0.1 0.1 0.1 0.1
Amortization of actuarial loss 3.8 3.5 7.6 7.0
Total 2.9 3.3 5.8 6.6
IPL [Member] | Other Postretirement Benefits Plans [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.7 0.7 1.4 1.5
Interest cost 0.9 1.1 1.8 2.2
Expected return on plan assets (1.4) (1.3) (2.8) (2.6)
Amortization of prior service cost (credit) (1.5) (1.6) (3.1) (3.2)
Amortization of actuarial loss 0.7 0.9 1.4 1.8
Total (0.6) (0.2) (1.3) (0.3)
WPL [Member] | Defined benefit pension plans [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1.5 1.3 2.9 2.6
Interest cost 5.1 5.4 10.3 10.8
Expected return on plan assets (7.9) (7.4) (15.9) (14.8)
Amortization of prior service cost (credit) 0.1 0.1 0.2 0.1
Amortization of actuarial loss 4.3 3.9 8.6 7.9
Total 3.1 3.3 6.1 6.6
WPL [Member] | Other Postretirement Benefits Plans [Member]
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.6 0.7 1.2 1.4
Interest cost 0.9 1.1 1.7 2.1
Expected return on plan assets (0.3) (0.4) (0.6) (0.7)
Amortization of prior service cost (credit) (1.0) (0.9) (2.0) (1.9)
Amortization of actuarial loss 0.5 0.5 1.0 1.1
Total $ 0.7 $ 1.0 $ 1.3 $ 2.0