Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 842	$ 1,022	$ 2,940	$ 4,450
Interest cost	5,283	4,260	15,719	10,945
Expected return on plan assets	(6,421)	(7,076)	(19,292)	(19,274)
Amortization of net actuarial loss	0	0	0	0
Defined Benefit Plan, Amortization of Gain (Loss)	139	(1,542)	418	9,459
Net periodic benefit cost	(157)	(3,336)	(215)	5,580
Amount allocated to construction	(326)	(496)	(1,239)	(1,965)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	(483)	(3,832)	(1,454)	3,615
Amortization of Regulatory Balancing Account	675	675	4,757	4,757
Net amount charged to expense	192	(3,157)	3,303	8,372
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	27	34	83	145
Interest cost	247	213	743	543
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	0	(15)	0	(249)
Defined Benefit Plan, Amortization of Gain (Loss)	0	57	0	319
Net periodic benefit cost	274	289	826	758
Amount allocated to construction	(9)	(14)	(29)	(57)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	265	275	797	701
Amortization of Regulatory Balancing Account	0	0	0	0
Net amount charged to expense	$ 265	$ 275	$ 797	$ 701