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EMPLOYEE BENEFIT PLANS (Tables) | 6 Months Ended
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Jun. 30, 2018
Employee Benefit Plans [Line Items] |
Schedule of Net Benefit Costs [Table Text Block] | The following tables set forth the components of net periodic benefit cost for our pension and other postemployment 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 | $ | 3,230 |  | $ | 3,044 |  |  | $ | 6,460 |  | $ | 6,088 |
Interest cost (a) | 9,200 |  | 10,113 |  |  | 18,400 |  | 20,226 |
Expected return on assets (a) | (15,145 | ) | (14,624 | ) |  | (30,290 | ) | (29,248 | )
Amortization of net loss (a) | 9,978 |  | 9,027 |  |  | 19,956 |  | 18,054 |
Net periodic benefit cost | $ | 7,263 |  | $ | 7,560 |  |  | $ | 14,526 |  | $ | 15,120 |
(a) Upon adoption of ASU 2017-07 on January 1, 2018, these amounts are recognized as other income (expense) in the Consolidated Statements of Income. See Note 11 for additional detail of our other income (expense).
| Other Postemployment Benefits
| Three Months Ended |  | Six Months Ended
| June 30, |  | June 30,
| 2018 | 2017 |  | 2018 | 2017
| (Thousands of dollars)
Components of net periodic benefit (credit) cost |  |  |  |  |
Service cost | $ | 589 |  | $ | 627 |  |  | $ | 1,178 |  | $ | 1,254 |
Interest cost (a) | 2,279 |  | 2,472 |  |  | 4,558 |  | 4,944 |
Expected return on assets (a) | (3,571 | ) | (3,147 | ) |  | (7,142 | ) | (6,294 | )
Amortization of unrecognized prior service cost (a) | (1,142 | ) | (1,149 | ) |  | (2,284 | ) | (2,298 | )
Amortization of net loss (a) | 972 |  | 1,621 |  |  | 1,944 |  | 3,242 |
Net periodic benefit (credit) cost | $ | (873 | ) | $ | 424 |  |  | $ | (1,746 | ) | $ | 848 |
(a) Upon adoption of ASU 2017-07 on January 1, 2018, these amounts are recognized as other income (expense) in the Consolidated Statements of Income. See Note 11 for additional detail of our other income (expense).

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + 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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