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Employee Benefit Plans: (Tables)
 9 Months Ended
Sep. 30, 2025
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 for Registrant’s pension plan, postretirement medical benefit plan and SERP for the three and nine months ended September 30, 2025 and 2024 were as follows:
For The Three Months Ended September 30,
 Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202520242025202420252024
Components of Net Periodic Benefits Cost:      
Service cost$721 $792 $27 $32 $181 $358 
Interest cost2,691 2,580 23 25 485 426 
Expected return on plan assets(3,181)(3,004)(161)(142)— — 
Amortization of prior service cost 109 109 — — — — 
Amortization of actuarial (gain) loss— — (291)(278)— (4)
Net periodic benefits costs under accounting standards$340 $477 $(402)$(363)$666 $780 
For The Nine Months Ended September 30,
Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202520242025202420252024
Components of Net Periodic Benefits Cost:
Service cost$2,163 $2,492 $81 $96 $542 $1,074 
Interest cost8,073 7,680 68 74 1,455 1,278 
Expected return on plan assets(9,543)(9,022)(483)(426)— — 
Amortization of prior service cost327 326 — — — — 
Amortization of actuarial (gain) loss— — (874)(834)— (11)
Net periodic benefits costs under accounting standards$1,020 $1,476 $(1,208)$(1,090)$1,997 $2,341