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EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS (Tables) | 6 Months Ended
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Jun. 30, 2018
Defined Benefit Plan [Abstract] |
Components of net periodic benefit cost for pension and postretirement benefit plans | The following tables set forth the components of net periodic benefit cost for our pension and postretirement benefit plans for the periods indicated:
| Pension Benefits
| Three Months Ended |  | Six Months Ended
| June 30, |  | June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
| (Thousands of dollars)
Components of net periodic benefit cost |  |  |  |  |  |  |
Service cost | $ | 1,832 |  |  | $ | 1,722 |  |  | $ | 3,664 |  |  | $ | 3,444 |
Interest cost | 4,408 |  |  | 4,655 |  |  | 8,816 |  |  | 9,310 |
Expected return on plan assets | (5,969 | ) |  | (5,336 | ) |  | (11,938 | ) |  | (10,672 | )
Amortization of net loss | 4,258 |  |  | 3,392 |  |  | 8,516 |  |  | 6,784 |
Net periodic benefit cost | $ | 4,529 |  |  | $ | 4,433 |  |  | $ | 9,058 |  |  | $ | 8,866 |
| Postretirement Benefits
| Three Months Ended |  | Six Months Ended
| June 30, |  | June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
| (Thousands of dollars)
Components of net periodic benefit cost |  |  |  |  |  |  |
Service cost | $ | 211 |  |  | $ | 165 |  |  | $ | 422 |  |  | $ | 330 |
Interest cost | 527 |  |  | 565 |  |  | 1,054 |  |  | 1,130 |
Expected return on plan assets | (672 | ) |  | (564 | ) |  | (1,344 | ) |  | (1,128 | )
Amortization of prior service credit | (415 | ) |  | (415 | ) |  | (830 | ) |  | (830 | )
Amortization of net loss | 334 |  |  | 420 |  |  | 668 |  |  | 840 |
Net periodic benefit cost (income) | $ | (15 | ) |  | $ | 171 |  |  | $ | (30 | ) |  | $ | 342 |

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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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