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ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Tables) | 6 Months Ended
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Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items] |
Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] | The following table sets forth the effect of reclassifications from accumulated other comprehensive income (loss) in our Consolidated Statements of Income for the periods indicated:
|  | Three Months Ended |  | Six Months Ended |  | Affected Line Item in the
Details about Accumulated Other |  | June 30, |  | June 30, |  | Consolidated Statements
Comprehensive Income (Loss) Components |  | 2018 | 2017 |  | 2018 | 2017 |  | of Income
|  | (Thousands of dollars) |  |
Pension and other postemployment benefit plan obligations (a) |  |  |  |  |  |  |  |
Amortization of net loss |  | $ | 10,950 |  | $ | 10,648 |  |  | $ | 21,900 |  | $ | 21,296 |  |  |
Amortization of unrecognized prior service cost |  | (1,142 | ) | (1,149 | ) |  | (2,284 | ) | (2,298 | ) |  |
|  | 9,808 |  | 9,499 |  |  | 19,616 |  | 18,998 |  |  |
Regulatory adjustments (b) |  | (9,537 | ) | (9,289 | ) |  | (19,074 | ) | (18,579 | ) |  |
|  | 271 |  | 210 |  |  | 542 |  | 419 |  |  | Income before income taxes
|  | (68 | ) | (81 | ) |  | (419 | ) | (161 | ) |  | Income tax expense
Total reclassifications for the period |  | $ | 203 |  | $ | 129 |  |  | $ | 123 |  | $ | 258 |  |  | Net income
(a) These components of accumulated other comprehensive income (loss) are included in the computation of net periodic benefit cost. See Note 9 for additional detail of our net periodic benefit cost. (b) Regulatory adjustments represent pension and other postemployment benefit costs expected to be recovered through rates and are deferred as part of our regulatory assets. See Note 3 for additional disclosures of regulatory assets and liabilities.

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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 information about items reclassified out of accumulated other comprehensive income (loss). + References No definition available. + Details
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