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Components of Net Periodic Benefit Cost | 9 Months Ended
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Sep. 30, 2020
Components of net periodic benefit cost [Abstract] |
Components of Net Periodic Benefit Cost | Components of Net Periodic Benefit Cost
| Pension Benefits
| Three months ended September 30, |  | Nine months ended September 30, |  |
| 2020 |  | 2019 |  | 2020 |  | 2019 |  |  |  |
Service cost | $ | 1.3 |  |  | $ | 1.0 |  |  | $ | 3.7 |  |  | $ | 2.5 |  |  |  |  |
Interest cost | 2.9 |  |  | 3.3 |  |  | 8.5 |  |  | 10.0 |  |  |  |  |
Expected return on plan assets | (5.3) |  |  | (4.5) |  |  | (15.6) |  |  | (13.7) |  |  |  |  |
Amortization of prior service costs | 0.1 |  |  | 0.0 |  |  | 0.1 |  |  | 0.6 |  |  |  |  |
Amortization of net loss | 1.6 |  |  | 1.6 |  |  | 4.9 |  |  | 4.9 |  |  |  |  |
Net periodic benefit cost | $ | 0.6 |  |  | $ | 1.4 |  |  | $ | 1.6 |  |  | $ | 4.3 |  |  |  |  |
| Other Postretirement Benefits ("OPEB")
| Three months ended September 30, |  | Nine months ended September 30, |  |
| 2020 |  | 2019 |  | 2020 |  | 2019 |  |  |  |
Service cost | $ | 0.0 |  |  | $ | 0.0 |  |  | $ | 0.2 |  |  | $ | 0.8 |  |  |  |  |
Interest cost | 0.8 |  |  | 1.1 |  |  | 2.3 |  |  | 3.4 |  |  |  |  |
Amortization of prior service cost | (0.8) |  |  | (1.2) |  |  | (2.4) |  |  | (4.3) |  |  |  |  |
Amortization of net loss | 0.5 |  |  | 0.6 |  |  | 1.6 |  |  | 1.7 |  |  |  |  |
Net periodic benefit cost | $ | 0.5 |  |  | $ | 0.5 |  |  | $ | 1.7 |  |  | $ | 1.6 |  |  |  |  |

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