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Components of net periodic benefit cost | 9 Months Ended
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Sep. 30, 2017
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,
| 2017 |  | 2016 |  | 2017 |  | 2016
Service cost | $ | 1,096 |  |  | $ | 1,270 |  |  | $ | 3,288 |  |  | $ | 3,810 |
Interest cost | 3,327 |  |  | 3,478 |  |  | 9,982 |  |  | 10,440 |
Expected return on plan assets | (4,750 | ) |  | (4,813 | ) |  | (14,615 | ) |  | (14,445 | )
Amortization of prior service costs | 27 |  |  | 28 |  |  | 80 |  |  | 84 |
Amortization of net loss | 1,187 |  |  | 1,041 |  |  | 3,925 |  |  | 3,125 |
Net periodic benefit cost | $ | 887 |  |  | $ | 1,004 |  |  | $ | 2,660 |  |  | $ | 3,014 |
| Other Postretirement Benefits ("OPEB")
| Three months ended September 30, |  | Nine months ended September 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Service cost | $ | 219 |  |  | $ | 330 |  |  | $ | 657 |  |  | $ | 990 |
Interest cost | 1,382 |  |  | 1,469 |  |  | 5,726 |  |  | 4,364 |
Amortization of prior service cost | (695 | ) |  | (695 | ) |  | (1,731 | ) |  | (2,085 | )
Amortization of net loss | 958 |  |  | 878 |  |  | 940 |  |  | 2,633 |
Net periodic benefit cost | $ | 1,864 |  |  | $ | 1,982 |  |  | $ | 5,592 |  |  | $ | 5,902 |

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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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