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Pension and Postretirement Benefit Costs (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Components of net periodic benefit cost |  |  |  |
Net periodic pension (benefit) cost | $ 2.7 | $ 4.8 | $ 5.5 | $ 9.7
Defined benefit pension plans |  |  |  |
Components of net periodic benefit cost |  |  |  |
Service cost for benefits earned | 0.1 | 0.5 | 0.1 | 1.0
Interest cost on projected benefit obligation | 7.0 | 8.4 | 14.0 | 16.7
Expected return on plan assets | (7.4) | (7.8) | (14.8) | (15.6)
Net periodic pension (benefit) cost | $ (0.3) | 1.1 | $ (0.7) | 2.1
Other Defined Benefit Plan Information [Abstract] |  |  |  |
Funding threshold | 80.00% |  | 80.00% |
Postretirement benefit plans |  |  |  |
Components of net periodic postretirement benefits cost |  |  |  |
Service cost for benefits earned | $ 1.1 | 1.2 | $ 2.2 | 2.4
Interest cost on accumulated postretirement benefit obligation | 5.4 | 6.3 | 10.9 | 12.6
Expected return on plan assets | (0.4) | (0.1) | (0.8) | (0.2)
Amortization of prior service credit | (2.2) | (2.2) | (4.4) | (4.4)
Net periodic postretirement benefit cost | $ 3.9 | $ 5.2 | $ 7.9 | $ 10.4

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