v3.25.2
Employee Benefit Plans: (Tables)
 6 Months Ended
Jun. 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 six months ended June 30, 2025 and 2024 were as follows:
For The Three Months Ended June 30,
 Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202520242025202420252024
Components of Net Periodic Benefits Cost:      
Service cost$668 $850 $27 $32 $180 $358 
Interest cost2,676 2,550 21 26 485 426 
Expected return on plan assets(3,179)(3,009)(161)(142)— — 
Amortization of prior service cost 110 109 — — — — 
Amortization of actuarial (gain) loss— — (292)(293)— (3)
Net periodic benefits costs under accounting standards$275 $500 $(405)$(377)$665 $781 
For The Six Months Ended June 30,
Pension BenefitsOther
Postretirement
Benefits		SERP
(dollars in thousands)202520242025202420252024
Components of Net Periodic Benefits Cost:
Service cost$1,442 $1,700 $54 $64 $361 $716 
Interest cost5,382 5,100 45 49 970 852 
Expected return on plan assets(6,362)(6,018)(322)(284)— — 
Amortization of prior service cost218 217 — — — — 
Amortization of actuarial (gain) loss— — (583)(556)— (7)
Net periodic benefits costs under accounting standards$680 $999 $(806)$(727)$1,331 $1,561