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Components of net periodic benefit cost | 6 Months Ended
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Jun. 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 June 30, |  | Six months ended June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Service cost | $ | 1.0 |  |  | $ | 1.0 |  |  | $ | 2.1 |  |  | $ | 2.2 |
Interest cost | 3.0 |  |  | 3.3 |  |  | 6.1 |  |  | 6.7 |
Expected return on plan assets | (5.3 | ) |  | (5.1 | ) |  | (10.6 | ) |  | (9.9 | )
Amortization of prior service costs | 0.1 |  |  | 0.0 |  |  | 0.1 |  |  | 0.1 |
Amortization of net loss | 1.1 |  |  | 1.5 |  |  | 2.6 |  |  | 2.7 |
Net periodic benefit cost | $ | (0.1 | ) |  | $ | 0.7 |  |  | $ | 0.3 |  |  | $ | 1.8 |
| Other Postretirement Benefits ("OPEB")
| Three months ended June 30, |  | Six months ended June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Service cost | $ | 0.0 |  |  | $ | 0.2 |  |  | $ | 0.1 |  |  | $ | 0.4 |
Interest cost | (1.8 | ) |  | 3.0 |  |  | (0.7 | ) |  | 4.3 |
Amortization of prior service cost | (1.5 | ) |  | (0.7 | ) |  | (3.3 | ) |  | (1.0 | )
Amortization of net loss | 3.3 |  |  | (1.0 | ) |  | 4.4 |  |  | 0.0 |
Net periodic benefit cost | $ | 0.0 |  |  | $ | 1.5 |  |  | $ | 0.5 |  |  | $ | 3.7 |

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -URI http://asc.fasb.org/topic&trid=2235017 + Details
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