Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,049	$ 1,529	$ 2,098	$ 3,059
Interest cost	5,218	3,659	10,436	7,318
Expected return on plan assets	(6,210)	(6,427)	(12,871)	(12,854)
Amortization of net actuarial loss	0	0	0	0
Defined Benefit Plan, Amortization of Gain (Loss)	140	3,199	279	6,397
Net periodic benefit cost	197	1,960	(58)	3,920
Amount allocated to construction	(439)	(616)	(913)	(1,280)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	(242)	1,344	(971)	2,640
Amortization of Regulatory Balancing Account	1,281	1,281	4,082	4,082
Net amount charged to expense	1,039	2,625	3,111	6,722
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	28	47	56	94
Interest cost	248	179	496	359
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	0	(84)	0	(167)
Defined Benefit Plan, Amortization of Gain (Loss)	0	99	0	198
Net periodic benefit cost	276	241	552	484
Amount allocated to construction	(9)	(17)	(20)	(35)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	267	224	532	449
Amortization of Regulatory Balancing Account	0	0	0	0
Net amount charged to expense	$ 267	$ 224	$ 532	$ 449