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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, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Defined benefit pension plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  | $ 2.7 | $ 2.4 | $ 5.4 | $ 4.8
Interest cost |  | 10.8 | 12.4 | 21.6 | 24.9
Expected return on plan assets |  | (17.5) | (15.0) | (34.9) | (30.0)
Amortization of prior service credit |  | (0.1) | (0.1) | (0.3) | (0.3)
Amortization of actuarial loss |  | 8.6 | 9.1 | 17.2 | 18.2
Settlement losses | [1] | 0.0 | 0.0 | 4.2 | 0.0
Total |  | 4.5 | 8.8 | 13.2 | 17.6
OPEB Plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  | 0.9 | 0.9 | 1.7 | 1.7
Interest cost |  | 1.7 | 2.1 | 3.5 | 4.2
Expected return on plan assets |  | (1.4) | (1.3) | (2.7) | (2.5)
Amortization of prior service credit |  | 0.0 | (0.1) | 0.0 | (0.1)
Amortization of actuarial loss |  | 0.9 | 0.8 | 1.7 | 1.6
Settlement losses |  | 0.0 | 0.0 | 0.0 | 0.0
Total |  | 2.1 | 2.4 | 4.2 | 4.9
IPL [Member] | Defined benefit pension plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  | 1.7 | 1.5 | 3.4 | 3.0
Interest cost |  | 5.0 | 5.7 | 10.0 | 11.4
Expected return on plan assets |  | (8.2) | (7.1) | (16.3) | (14.1)
Amortization of prior service credit |  | 0.0 | 0.0 | (0.1) | (0.1)
Amortization of actuarial loss |  | 3.7 | 4.0 | 7.4 | 7.9
Total |  | 2.2 | 4.1 | 4.4 | 8.1
IPL [Member] | OPEB Plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  | 0.4 | 0.4 | 0.7 | 0.7
Interest cost |  | 0.7 | 0.9 | 1.4 | 1.7
Expected return on plan assets |  | (0.9) | (0.9) | (1.9) | (1.8)
Amortization of prior service credit |  | 0.0 | 0.0 | 0.0 | 0.0
Amortization of actuarial loss |  | 0.2 | 0.3 | 0.5 | 0.7
Total |  | 0.4 | 0.7 | 0.7 | 1.3
WPL [Member] | Defined benefit pension plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  | 0.9 | 0.8 | 1.9 | 1.7
Interest cost |  | 4.7 | 5.4 | 9.4 | 10.8
Expected return on plan assets |  | (7.6) | (6.6) | (15.2) | (13.1)
Amortization of prior service credit |  | 0.0 | 0.0 | (0.1) | (0.1)
Amortization of actuarial loss |  | 4.1 | 4.4 | 8.2 | 8.8
Total |  | 2.1 | 4.0 | 4.2 | 8.1
WPL [Member] | OPEB Plans [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Service cost |  | 0.3 | 0.3 | 0.6 | 0.6
Interest cost |  | 0.7 | 0.9 | 1.4 | 1.7
Expected return on plan assets |  | (0.1) | (0.2) | (0.3) | (0.3)
Amortization of prior service credit |  | 0.0 | (0.1) | 0.0 | (0.1)
Amortization of actuarial loss |  | 0.5 | 0.4 | 1.0 | 0.8
Total |  | $ 1.4 | $ 1.3 | $ 2.7 | $ 2.7
[1] | Settlement losses related to payments made to retired executives of Alliant Energy.

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