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Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Defined benefit pension plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 3.1 $ 3.1 $ 6.2 $ 6.3
Interest cost 12.8 13.2 25.6 26.5
Expected return on plan assets (16.4) (16.4) (32.8) (32.8)
Amortization of prior service cost (credit) (0.1) 0.0 (0.2) (0.1)
Amortization of actuarial loss 9.4 9.4 18.8 18.7
Total 8.8 9.3 17.6 18.6
OPEB Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1.3 1.3 2.5 2.6
Interest cost 2.1 2.4 4.3 4.7
Expected return on plan assets (1.6) (1.5) (3.1) (3.0)
Amortization of prior service cost (credit) 0.0 (1.1) (0.1) (2.1)
Amortization of actuarial loss 0.9 1.2 1.9 2.4
Total 2.7 2.3 5.5 4.6
IPL [Member] | Defined benefit pension plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1.9 1.9 3.7 3.8
Interest cost 5.8 6.2 11.7 12.3
Expected return on plan assets (7.7) (7.8) (15.4) (15.5)
Amortization of prior service cost (credit) (0.1) (0.1) (0.1) (0.1)
Amortization of actuarial loss 4.1 4.1 8.1 8.2
Total 4.0 4.3 8.0 8.7
IPL [Member] | OPEB Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.6 0.6 1.1 1.2
Interest cost 0.9 0.9 1.8 1.9
Expected return on plan assets (1.1) (1.2) (2.2) (2.2)
Amortization of prior service cost (credit) 0.0 (0.6) 0.0 (1.3)
Amortization of actuarial loss 0.5 0.7 1.0 1.3
Total 0.9 0.4 1.7 0.9
WPL [Member] | Defined benefit pension plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1.2 1.2 2.4 2.4
Interest cost 5.4 5.6 10.9 11.2
Expected return on plan assets (7.1) (7.1) (14.2) (14.2)
Amortization of prior service cost (credit) 0.0 0.0 0.0 0.1
Amortization of actuarial loss 4.7 4.4 9.3 8.8
Total 4.2 4.1 8.4 8.3
WPL [Member] | OPEB Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.4 0.5 0.9 1.0
Interest cost 0.8 1.0 1.7 1.9
Expected return on plan assets (0.2) (0.2) (0.4) (0.4)
Amortization of prior service cost (credit) 0.0 (0.2) (0.1) (0.4)
Amortization of actuarial loss 0.4 0.4 0.8 0.9
Total $ 1.4 $ 1.5 $ 2.9 $ 3.0