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Employee Benefit Plans: (Tables) | 9 Months Ended
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Sep. 30, 2018
Retirement Benefits [Abstract] |
Schedule of components of net periodic benefit costs, before allocation to the overhead pool, for Registrant's pension plan, postretirement plan, and SERP | The components of net periodic benefit costs, before allocation to the overhead pool, for Registrant’s pension plan, postretirement plan and the supplemental executive retirement plan ("SERP") for the three and nine months ended September 30, 2018 and 2017 are as follows:
|  | For The Three Months Ended September 30,
|  | Pension Benefits |  | Other Postretirement Benefits |  | SERP
(dollars in thousands) |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Components of Net Periodic Benefits Cost: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service cost |  | $ | 1,335 |  |  | $ | 1,250 |  |  | $ | 48 |  |  | $ | 53 |  |  | $ | 274 |  |  | $ | 232 |
Interest cost |  | 1,912 |  |  | 1,976 |  |  | 75 |  |  | 73 |  |  | 222 |  |  | 223 |
Expected return on plan assets |  | (2,793 | ) |  | (2,428 | ) |  | (123 | ) |  | (107 | ) |  | — |  |  | — |
Amortization of prior service cost (benefit) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | 3 |
Amortization of actuarial (gain) loss |  | 314 |  |  | 231 |  |  | (212 | ) |  | (242 | ) |  | 262 |  |  | 194 |
Net periodic pension cost under accounting standards |  | 768 |  |  | 1,029 |  |  | (212 | ) |  | (223 | ) |  | 758 |  |  | 652 |
Regulatory adjustment — deferred |  | — |  |  | 266 |  |  | — |  |  | — |  |  | — |  |  | — |
Total expense recognized, before surcharges and allocation to overhead pool |  | $ | 768 |  |  | $ | 1,295 |  |  | $ | (212 | ) |  | $ | (223 | ) |  | $ | 758 |  |  | $ | 652 |
|  | For The Nine Months Ended September 30,
|  | Pension Benefits |  | Other Postretirement Benefits |  | SERP
(dollars in thousands) |  | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Components of Net Periodic Benefits Cost: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service cost |  | $ | 4,005 |  |  | $ | 3,750 |  |  | $ | 162 |  |  | $ | 171 |  |  | $ | 822 |  |  | $ | 696 |
Interest cost |  | 5,736 |  |  | 5,928 |  |  | 219 |  |  | 243 |  |  | 666 |  |  | 669 |
Expected return on plan assets |  | (8,379 | ) |  | (7,278 | ) |  | (369 | ) |  | (351 | ) |  | — |  |  | — |
Amortization of prior service cost (benefit) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | 9 |
Amortization of actuarial (gain) loss |  | 942 |  |  | 693 |  |  | (576 | ) |  | (582 | ) |  | 786 |  |  | 582 |
Net periodic pension cost under accounting standards |  | 2,304 |  |  | 3,093 |  |  | (564 | ) |  | (519 | ) |  | 2,274 |  |  | 1,956 |
Regulatory adjustment — deferred |  | — |  |  | 791 |  |  | — |  |  | — |  |  | — |  |  | — |
Total expense recognized, before surcharges and allocation to overhead pool |  | $ | 2,304 |  |  | $ | 3,884 |  |  | $ | (564 | ) |  | $ | (519 | ) |  | $ | 2,274 |  |  | $ | 1,956 |

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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=82843861&loc=d3e1928-114920 + Details
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