Employee Benefit Plans - Components of Net Periodic Pension Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets	5.50%	5.50%	5.75%
Non-union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets	3.90%	3.90%	4.00%
Pension Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 3,196	$ 5,644	$ 6,316
Interest cost	10,142	7,401	6,833
Expected return on plan assets	(10,483)	(13,166)	(12,541)
Amortization of prior service cost (credit)	434	434	434
Amortization of actuarial (gain) loss	0	0	3,817
Net periodic pension cost under accounting standards	3,289	313	4,859
Regulatory adjustment	(281)	0	(1,277)
Total expense recognized, before surcharges and allocation to overhead pool	$ 3,008	$ 313	$ 3,582
Weighted-average assumptions used to determine net periodic cost:
Discount rate	5.41%	2.89%	2.55%
Expected long-term return on plan assets	5.75%	5.75%	6.00%
Pension Benefits | Minimum
Weighted-average assumptions used to determine net periodic cost:
Rate of compensation increase	2.50%	2.50%	2.50%
Pension Benefits | Maximum
Weighted-average assumptions used to determine net periodic cost:
Rate of compensation increase	7.00%	7.00%	7.00%
Post-Retirement Medical Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 130	$ 129	$ 149
Interest cost	106	60	110
Expected return on plan assets	(477)	(587)	(537)
Amortization of prior service cost (credit)	0	0	0
Amortization of actuarial (gain) loss	(969)	(1,734)	(1,417)
Net periodic pension cost under accounting standards	(1,210)	(2,132)	(1,695)
Regulatory adjustment	0	0	0
Total expense recognized, before surcharges and allocation to overhead pool	$ (1,210)	$ (2,132)	$ (1,695)
Weighted-average assumptions used to determine net periodic cost:
Discount rate	5.34%	2.46%	2.20%