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Retirement Plans and Other Postretirement Benefits (Tables) | 3 Months Ended
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Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract] |
Schedule of net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset) | The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged to the regulatory asset or liability) (dollars in thousands):
| Pension Benefits |  | Other Benefits
| Three Months Ended March 31, |  | Three Months Ended March 31,
| 2017 |  | 2016 |  | 2017 |  | 2016
Service cost — benefits earned during the period | $ | 13,760 |  |  | $ | 14,266 |  |  | $ | 4,358 |  |  | $ | 3,937 |
Interest cost on benefit obligation | 32,701 |  |  | 32,945 |  |  | 7,565 |  |  | 7,341 |
Expected return on plan assets | (43,710 | ) |  | (43,792 | ) |  | (13,350 | ) |  | (9,122 | )
Amortization of: |  |  |  |  |  |  |  |  |  |
Prior service cost (credit) | 20 |  |  | 132 |  |  | (9,461 | ) |  | (9,471 | )
Net actuarial loss | 12,489 |  |  | 9,731 |  |  | 1,454 |  |  | 946 |
Net periodic benefit cost | $ | 15,260 |  |  | $ | 13,282 |  |  | $ | (9,434 | ) |  | $ | (6,369 | )
Portion of cost charged to expense | $ | 7,568 |  |  | $ | 6,519 |  |  | $ | (4,678 | ) |  | $ | (3,126 | )

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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=65897934&loc=d3e1928-114920 + Details
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