Employee Benefit Plans - Pension Benefits and Post-Retirement Medical Benefits Recognized on Balance Sheet (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Amounts recognized on the balance sheets:
Non-current liabilities	$ (32,652)	$ (33,636)
Pension Benefits
Amounts recognized on the balance sheets:
Non-current assets	2,342	0
Current liabilities	0	0
Non-current liabilities	0	(3,772)
Net amount recognized	2,342	(3,772)
Amounts recognized in regulatory assets (liabilities) consist of:
Prior service cost (credit)	1,454	1,889
Net loss (gain)	(1,899)	4,123
Regulatory assets (liabilities)	(445)	6,012	$ 25,691
Prefunded plan costs	(1,897)	(2,240)
Net liability (asset) recognized	(2,342)	3,772
Changes in plan assets and benefit obligations recognized in regulatory assets (liabilities):
Regulatory asset (liability) at beginning of year	6,012	25,691
Net loss (gain)	(6,023)	(19,245)
New prior service cost	0	0
Amortization of prior service (cost) credit	(434)	(434)
Amortization of net gain (loss)	0	0
Total change in regulatory asset (liability)	(6,457)	(19,679)
Regulatory asset (liability) at end of year	(445)	6,012	25,691
Net periodic pension costs	3,289	313	$ 4,859
Change in regulatory asset (liability)	(6,457)	(19,679)
Total recognized in net periodic pension cost and regulatory asset (liability)	(3,168)	(19,366)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	202,963	190,678
Accumulated benefit obligation	192,986	181,376
Fair value of plan assets	$ 205,305	$ 186,906
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate	5.16%	5.41%
Pension Benefits | Minimum
Weighted-average assumptions used to determine benefit obligations at December 31:
Rate of compensation increase	2.50%	2.50%	2.50%
Pension Benefits | Maximum
Weighted-average assumptions used to determine benefit obligations at December 31:
Rate of compensation increase	7.00%	7.00%	7.00%
Post-Retirement Medical Benefits
Amounts recognized on the balance sheets:
Non-current assets	$ 10,862	$ 9,226
Current liabilities	0	0
Non-current liabilities	0	0
Net amount recognized	10,862	9,226
Amounts recognized in regulatory assets (liabilities) consist of:
Prior service cost (credit)	0	0
Net loss (gain)	(6,272)	(5,846)
Regulatory assets (liabilities)	(6,272)	(5,846)	$ (9,839)
Prefunded plan costs	(4,590)	(3,380)
Net liability (asset) recognized	(10,862)	(9,226)
Changes in plan assets and benefit obligations recognized in regulatory assets (liabilities):
Regulatory asset (liability) at beginning of year	(5,846)	(9,839)
Net loss (gain)	(1,395)	2,259
New prior service cost	0	0
Amortization of prior service (cost) credit	0	0
Amortization of net gain (loss)	969	1,734
Total change in regulatory asset (liability)	(426)	3,993
Regulatory asset (liability) at end of year	(6,272)	(5,846)	(9,839)
Net periodic pension costs	(1,210)	(2,132)	$ (1,695)
Change in regulatory asset (liability)	(426)	3,993
Total recognized in net periodic pension cost and regulatory asset (liability)	(1,636)	1,861
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	1,965	2,014
Fair value of plan assets	$ 12,827	$ 11,240
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate	5.04%	5.34%