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EMPLOYEE BENEFIT PLANS (Notes) | 6 Months Ended
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Jun. 30, 2018
Employee Benefit Plans [Line Items] |
EMPLOYEE BENEFIT PLANS |
9. | EMPLOYEE BENEFIT PLANS
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). We recover qualified pension benefit plan and other postemployment benefit plan costs through rates charged to our customers. Certain regulatory authorities require that the recovery of these costs be based on specific guidelines. The difference between these regulatory-based amounts and the periodic benefit cost calculated pursuant to GAAP is deferred as a regulatory asset or liability and amortized to expense over periods in which this difference will be recovered in rates, as authorized by the applicable regulatory authorities. Regulatory deferrals related to net periodic benefit cost were not material for the three and six months ended June 30, 2018. Upon adoption of ASU 2017-07 on January 1, 2018, we continue to capitalize all eligible service cost and non-service cost components under the accounting requirements of Topic 980 (Regulated Operations) for rate regulated entities. Our consolidated balance sheets reflect the capitalized non-service cost components as a regulatory asset. See Note 3 of the Notes to the Consolidated Financial Statements in this Quarterly Report for additional information.

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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 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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