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Employee Benefit Plans: (Tables)
 9 Months Ended
Sep. 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 nine months ended September 30, 2024 and 2023 were as follows:
For The Three Months Ended September 30,
 Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202420232024202320242023
Components of Net Periodic Benefits Cost:      
Service cost$792 $705 $32 $33 $358 $312 
Interest cost2,580 2,581 25 26 426 411 
Expected return on plan assets(3,004)(2,617)(142)(119)— — 
Amortization of prior service cost 109 110 — — — — 
Amortization of actuarial (gain) loss— — (278)(242)(4)(8)
Net periodic benefits costs under accounting standards477 779 (363)(302)780 715 
Regulatory adjustments - deferred— (27)— — — — 
Total expense (benefit) recognized, before surcharges and allocation to overhead pool$477 $752 $(363)$(302)$780 $715 
For The Nine Months Ended September 30,
Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202420232024202320242023
Components of Net Periodic Benefits Cost:
Service cost$2,492 $2,397 $96 $99 $1,074 $936 
Interest cost7,680 7,607 74 77 1,278 1,233 
Expected return on plan assets(9,022)(7,863)(426)(358)— — 
Amortization of prior service cost326 326 — — — — 
Amortization of actuarial (gain) loss— — (834)(724)(11)(24)
Net periodic benefits costs under accounting standards1,476 2,467 (1,090)(906)2,341 2,145 
Regulatory adjustments - deferred— (211)— — — — 
Total expense (benefit) recognized, before surcharges and allocation to overhead pool$1,476 $2,256 $(1,090)$(906)$2,341 $2,145