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Components of net periodic benefit cost | 9 Months Ended
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Sep. 30, 2018
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,
| 2018 |  | 2017 |  | 2018 |  | 2017
Service cost | $ | 1.1 |  |  | $ | 1.1 |  |  | $ | 3.2 |  |  | $ | 3.3 |
Interest cost | 3.1 |  |  | 3.3 |  |  | 9.2 |  |  | 10.0 |
Expected return on plan assets | (5.3 | ) |  | (4.8 | ) |  | (15.8 | ) |  | (14.6 | )
Amortization of prior service costs | 0.0 |  | 0.0 |  |  | 0.1 |  |  | 0.1 |
Amortization of net loss | 1.3 |  |  | 1.2 |  |  | 3.9 |  |  | 3.9 |
Net periodic benefit cost | $ | 0.2 |  |  | $ | 0.8 |  |  | $ | 0.6 |  |  | $ | 2.7 |
| Other Postretirement Benefits ("OPEB")
| Three months ended September 30, |  | Nine months ended September 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Service cost | $ | 0.1 |  |  | $ | 0.2 |  |  | $ | 0.2 |  |  | $ | 0.7 |
Interest cost | 1.0 |  |  | 1.4 |  |  | 0.4 |  |  | 5.7 |
Amortization of prior service cost | (1.8 | ) |  | (0.7 | ) |  | (5.1 | ) |  | (1.7 | )
Amortization of net loss | 1.0 |  |  | 1.0 |  |  | 5.4 |  |  | 0.9 |
Net periodic benefit cost | $ | 0.3 |  |  | $ | 1.9 |  |  | $ | 0.9 |  |  | $ | 5.6 |

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- Definition Components of net periodic benefit cost [Abstract] + References No definition available. + Details
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- Definition The entire disclosure for pension and other postretirement benefits. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -URI http://asc.fasb.org/topic&trid=2235017 + Details
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