Employee Benefit Plans - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 278,286	$ 367,134
Service cost	0	0	$ 0
Interest cost	13,521	9,396	8,767
Plan participants' contributions	0	0
Actuarial loss/(gain)	5,395	(76,130)
Benefits paid	(21,616)	(22,114)
Benefit obligation at end of year	275,586	278,286	367,134
Change in net plan assets:
Fair value of plan assets at beginning of year	242,031	333,764
Actual return on plan assets	20,576	(69,619)
Employer contribution	7,567	0
Plan participants' contributions	0	0
Benefits paid	(21,616)	(22,114)
Fair value of net plan assets at end of year	248,558	242,031	333,764
Funded status - (under) over	(27,028)	(36,255)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	0	0
Noncurrent liabilities - other	27,028	36,255
Benefit obligation (liabilities) assets - net amount recognized	(27,028)	(36,255)
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss (gain)	32,273	32,378
Prior service credit	0	0
Total	32,273	32,378
Amounts recognized in regulatory assets or liabilities:
Actuarial loss (gain)	140,232	141,207
Prior service credit	0	0
Total	140,232	141,207
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	40,315	53,981
Service cost	534	894	1,033
Interest cost	1,956	1,383	1,370
Plan participants' contributions	479	566
Actuarial loss/(gain)	(215)	(13,083)
Benefits paid	(3,479)	(3,426)
Benefit obligation at end of year	39,590	40,315	53,981
Change in net plan assets:
Fair value of plan assets at beginning of year	76,640	99,844
Actual return on plan assets	5,518	(20,419)
Employer contribution	76	75
Plan participants' contributions	479	566
Benefits paid	(3,479)	(3,426)
Fair value of net plan assets at end of year	79,234	76,640	$ 99,844
Funded status - (under) over	39,644	36,325
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	39,644	36,325
Noncurrent liabilities - other	0	0
Benefit obligation (liabilities) assets - net amount recognized	39,644	36,325
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss (gain)	(3,515)	(2,923)
Prior service credit	(115)	(289)
Total	(3,630)	(3,212)
Amounts recognized in regulatory assets or liabilities:
Actuarial loss (gain)	(1,146)	(1,439)
Prior service credit	(2,619)	(3,796)
Total	$ (3,765)	$ (5,235)