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Retirement Plans (Tables) | 6 Months Ended
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Jun. 30, 2017
Retirement Plans |
Defined Benefit Plan Disclosure [Line Items] |
Benefit costs | Our pension costs, which include costs for the qualified TRP plan and the nonqualified SERP plan, are presented in the following table (in thousands):
| Quarter ended June 30, |  | Six months ended June 30,
| 2017 |  | 2016 |  | 2017 |  | 2016
Service cost-benefits earned during the period | $ | 311 |  |  | $ | 158 |  |  | $ | 436 |  |  | $ | 408 |
Interest cost on benefit obligation | 6,056 |  |  | 6,837 |  |  | 11,981 |  |  | 13,187 |
Expected return on plan assets | (6,511 | ) |  | (6,632 | ) |  | (13,161 | ) |  | (13,382 | )
Amortization of prior service cost | 167 |  |  | 190 |  |  | 317 |  |  | 340 |
Amortization of actuarial loss | 2,187 |  |  | 2,194 |  |  | 4,162 |  |  | 3,894 |
Expense for company-sponsored retirement plans | $ | 2,210 |  |  | $ | 2,747 |  |  | $ | 3,735 |  |  | $ | 4,447 |

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- Definition Tabular disclosure of one or more of the entity's defined benefit pension plans or one or more other defined benefit postretirement plans, separately for pension plans and other postretirement benefit plans including the entity's schedule of fair value of plan assets for defined benefit or other postretirement plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2439-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2410-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=77993564&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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