Pension Plan and Employee Benefits - Benefit Obligation, Plan Assets, and Funded Status (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 period	$ 500,869	$ 680,417
Service cost	4,977	8,589	$ 10,516
Interest cost	26,423	19,841	18,668
Plan participants’ contributions	0	0
Actuarial loss (gain)	26,300	(174,733)
Expenses paid	(3,018)	(3,744)
Benefits paid	(30,069)	(29,501)
Benefit obligation at end of period	525,482	500,869	680,417
Change in plan assets
Fair value of plan assets at beginning of period	402,285	527,427
Actual (loss) gain return on plan assets	38,761	(91,897)
Employer contributions	200	0
Expenses paid	(3,018)	(3,744)
Benefits paid	(30,069)	(29,501)
Fair value of plan assets at end of period	408,159	402,285	527,427
Unfunded status	(117,323)	(98,584)
Accumulated benefit obligation	505,508	481,398
OTHER BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period	43,306	55,257
Service cost	1,472	2,204	2,425
Interest cost	2,285	1,484	1,283
Plan participants’ contributions	0	1,904
Actuarial loss (gain)	(5,110)	(10,289)
Expenses paid	0	0
Benefits paid	(5,117)	(7,254)
Benefit obligation at end of period	47,056	43,306	55,257
Change in plan assets
Fair value of plan assets at beginning of period	0	0
Actual (loss) gain return on plan assets	0	0
Employer contributions	0	0
Expenses paid	0	0
Benefits paid	0	0
Fair value of plan assets at end of period	0	0	0
Unfunded status	(47,056)	(43,306)
SERP BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period	68,427	93,179
Service cost	142	227	232
Interest cost	3,604	2,679	2,538
Actuarial loss (gain)	(345)	(22,097)
Benefits paid	(4,366)	(5,561)
Benefit obligation at end of period	67,462	68,427	$ 93,179
Change in plan assets
Accumulated benefit obligation	$ 67,462	$ 68,427