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Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Defined Benefit Pension Plans [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost |  | $ 9.7 | $ 12.1 | $ 12.5
Interest cost |  | 49.8 | 46.8 | 51.0
Expected return on plan assets | [1] | (60.1) | (69.7) | (65.5)
Amortization of prior service cost (credit) |  | (0.7) | (0.7) | (0.4)
Amortization of actuarial loss | [2] | 36.4 | 35.2 | 37.6
Settlement losses | [3] | 0.0 | 0.0 | 0.9
Total |  | 35.1 | 23.7 | 36.1
Other Postretirement Benefits Plans [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost |  | 3.3 | 4.2 | 5.0
Interest cost |  | 8.5 | 7.7 | 8.6
Expected return on plan assets | [1] | (5.0) | (6.0) | (6.1)
Amortization of prior service cost (credit) | [4] | (0.2) | (0.2) | (0.2)
Amortization of actuarial loss | [2] | 3.3 | 3.4 | 3.8
Settlement losses |  | 0.0 | 0.0 | 0.0
Total |  | 9.9 | 9.1 | 11.1
IPL [Member] | Defined Benefit Pension Plans [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost |  | 6.0 | 7.4 | 7.3
Interest cost |  | 22.7 | 21.4 | 23.5
Expected return on plan assets | [1] | (28.1) | (32.6) | (30.8)
Amortization of prior service cost (credit) |  | (0.2) | (0.2) | (0.2)
Amortization of actuarial loss | [2] | 15.7 | 14.9 | 16.1
Total |  | 16.1 | 10.9 | 15.9
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost |  | 1.3 | 1.7 | 2.1
Interest cost |  | 3.4 | 3.1 | 3.5
Expected return on plan assets | [1] | (3.6) | (4.4) | (4.3)
Amortization of prior service cost (credit) | [4] | 0.0 | 0.0 | 0.0
Amortization of actuarial loss | [2] | 1.5 | 1.3 | 2.0
Total |  | 2.6 | 1.7 | 3.3
WPL [Member] | Defined Benefit Pension Plans [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost |  | 3.5 | 4.4 | 4.9
Interest cost |  | 21.5 | 20.2 | 21.8
Expected return on plan assets | [1] | (26.2) | (30.4) | (28.5)
Amortization of prior service cost (credit) |  | (0.2) | (0.1) | 0.1
Amortization of actuarial loss | [2] | 17.6 | 17.2 | 18.5
Total |  | 16.2 | 11.3 | 16.8
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service cost |  | 1.3 | 1.6 | 1.9
Interest cost |  | 3.3 | 3.1 | 3.4
Expected return on plan assets | [1] | (0.6) | (0.6) | (0.8)
Amortization of prior service cost (credit) | [4] | (0.2) | (0.2) | (0.2)
Amortization of actuarial loss | [2] | 1.6 | 2.0 | 1.6
Total |  | $ 5.4 | $ 5.9 | $ 5.9
[1] | The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
[2] | Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.
[3] | Settlement losses related to payments made to retired executives of Alliant Energy.
[4] | Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.

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