Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 980	$ 1,049
Interest cost	5,160	5,218
Expected return on plan assets	(5,935)	(6,661)
Defined Benefit Plan, Amortization of Gain (Loss)	1,495	139
Net periodic benefit cost	1,700	(255)
Amount allocated to construction	(425)	(474)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	1,275	(729)
Amortization of Regulatory Balancing Account	2,801	2,801
Net amount charged to expense	4,076	2,072
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	28
Interest cost	248	248
Expected return on plan assets	0	0
Defined Benefit Plan, Amortization of Gain (Loss)	0	0
Net periodic benefit cost	273	276
Amount allocated to construction	(8)	(11)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	265	265
Amortization of Regulatory Balancing Account	0	0
Net amount charged to expense	$ 265	$ 265