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Components of net periodic benefit cost (Tables) | 9 Months Ended
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Sep. 30, 2017
Components of net periodic benefit cost [Abstract] |
Schedule 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 Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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