| 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, | | 2020 | 2019 | | 2020 | 2019 | | (Thousands of dollars) | Components of net periodic benefit cost | | | | | | Service cost | $ | 3,217 |
| $ | 3,008 |
| | $ | 6,434 |
| $ | 6,016 |
| Interest cost | 8,545 |
| 10,168 |
| | 17,090 |
| 20,336 |
| Expected return on assets | (15,280 | ) | (15,485 | ) | | (30,560 | ) | (30,970 | ) | Amortization of net loss | 10,580 |
| 8,260 |
| | 21,160 |
| 16,520 |
| Net periodic benefit cost | $ | 7,062 |
| $ | 5,951 |
| | $ | 14,124 |
| $ | 11,902 |
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| | | | | | | | | | | | | | | | Other Postemployment Benefits | | Three Months Ended | | Six Months Ended | | June 30, | | June 30, | | 2020 | 2019 | | 2020 | 2019 | | (Thousands of dollars) | Components of net periodic benefit cost (credit) | | | | | | Service cost | $ | 423 |
| $ | 434 |
| | $ | 846 |
| $ | 868 |
| Interest cost | 1,889 |
| 2,329 |
| | 3,778 |
| 4,658 |
| Expected return on assets | (3,867 | ) | (3,147 | ) | | (7,734 | ) | (6,294 | ) | Amortization of unrecognized prior service credit | (29 | ) | (168 | ) | | (58 | ) | (336 | ) | Amortization of net loss | 43 |
| 561 |
| | 86 |
| 1,122 |
| Net periodic benefit cost (credit) | $ | (1,541 | ) | $ | 9 |
| | $ | (3,082 | ) | $ | 18 |
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