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Employee Benefit Plans: (Tables)
 6 Months Ended
Jun. 30, 2024
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 six months ended June 30, 2024 and 2023 were as follows:
For The Three Months Ended June 30,
 Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202420232024202320242023
Components of Net Periodic Benefits Cost:      
Service cost$850 $846 $32 $33 $358 $312 
Interest cost2,550 2,513 26 26 426 411 
Expected return on plan assets(3,009)(2,623)(142)(119)— — 
Amortization of prior service cost 109 108 — — — — 
Amortization of actuarial (gain) loss— — (293)(242)(3)(8)
Net periodic benefits costs under accounting standards500 844 (377)(302)781 715 
Regulatory adjustments - deferred— (92)— — — — 
Total expense (benefit) recognized, before surcharges and allocation to overhead pool$500 $752 $(377)$(302)$781 $715 
For The Six Months Ended June 30,
Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202420232024202320242023
Components of Net Periodic Benefits Cost:
Service cost$1,700 $1,692 $64 $66 $716 $624 
Interest cost5,100 5,026 49 51 852 822 
Expected return on plan assets(6,018)(5,246)(284)(239)— — 
Amortization of prior service cost217 216 — — — — 
Amortization of actuarial (gain) loss— — (556)(482)(7)(16)
Net periodic benefits costs under accounting standards999 1,688 (727)(604)1,561 1,430 
Regulatory adjustments - deferred— (184)— — — — 
Total expense (benefit) recognized, before surcharges and allocation to overhead pool$999 $1,504 $(727)$(604)$1,561 $1,430