Employee Benefit Plans - Components of Net Periodic Pension Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets	5.50%	5.75%	6.00%
Non-union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets	3.90%	4.00%	4.20%
Pension Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 5,644	$ 6,316	$ 5,558
Interest cost	7,401	6,833	7,880
Expected return on plan assets	(13,166)	(12,541)	(11,798)
Amortization of prior service cost (credit)	434	434	435
Amortization of actuarial (gain) loss	0	3,817	1,935
Net periodic pension cost under accounting standards	313	4,859	4,010
Regulatory adjustment	0	(1,277)	(483)
Total expense recognized, before surcharges and allocation to overhead pool	$ 313	$ 3,582	$ 3,527
Weighted-average assumptions used to determine net periodic cost:
Discount rate	2.89%	2.55%	3.43%
Expected long-term return on plan assets	5.75%	6.00%	6.25%
Pension Benefits | Minimum
Weighted-average assumptions used to determine net periodic cost:
Rate of compensation increase	3.00%	3.00%	3.00%
Pension Benefits | Maximum
Weighted-average assumptions used to determine net periodic cost:
Rate of compensation increase	8.00%	8.00%	8.00%
Post-Retirement Medical Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 129	$ 149	$ 171
Interest cost	60	110	208
Expected return on plan assets	(587)	(537)	(510)
Amortization of prior service cost (credit)	0	0	0
Amortization of actuarial (gain) loss	(1,734)	(1,417)	(977)
Net periodic pension cost under accounting standards	(2,132)	(1,695)	(1,108)
Regulatory adjustment	0	0	0
Total expense recognized, before surcharges and allocation to overhead pool	$ (2,132)	$ (1,695)	$ (1,108)
Weighted-average assumptions used to determine net periodic cost:
Discount rate	2.46%	2.20%	3.12%