Pension Plan and Employee Benefits - Benefit Obligation, Plan Assets, and Funded Status (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
PENSION BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period			$ 686,384	$ 610,323
Service cost			10,516	9,820	$ 8,414
Interest cost			18,668	20,816	22,485
Plan participants’ contributions			0	0
Actuarial (gain) loss			(9,823)	71,708
Expenses paid			(3,306)	(2,661)
Benefits paid			(25,292)	(23,622)
Special/contractual termination benefits			3,270	0	0
Benefit obligation at end of period	$ 686,384		680,417	686,384	610,323
Change in plan assets
Fair value of plan assets at beginning of period			516,120	460,097
Actual return on plan assets			39,905	66,557
Employer contributions	15,800	$ 12,300	0	15,750
Expenses paid			(3,306)	(2,662)
Benefits paid			(25,292)	(23,622)
Fair value of plan assets at end of period	516,120		527,427	516,120	460,097
Unfunded status	(170,264)		(152,990)	(170,264)
Accumulated benefit obligation	636,199		640,490	636,199
OTHER BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period			56,331	52,722
Service cost			2,425	2,153	1,191
Interest cost			1,283	1,651	1,646
Plan participants’ contributions			0	1,289
Actuarial (gain) loss			(81)	4,221
Expenses paid			0	0
Benefits paid			(4,701)	(5,705)
Special/contractual termination benefits			0	0	0
Benefit obligation at end of period	56,331		55,257	56,331	52,722
Change in plan assets
Fair value of plan assets at beginning of period			0	0
Actual 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	0
Unfunded status	(56,331)		(55,257)	(56,331)
SERP BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period			97,225	89,128
Service cost			232	399	330
Interest cost			2,538	2,932	3,326
Actuarial (gain) loss			(1,932)	9,621
Benefits paid			(4,884)	(4,590)
Plan amendments			0	(265)
Benefit obligation at end of period	97,225		93,179	97,225	$ 89,128
Change in plan assets
Accumulated benefit obligation	$ 97,225		$ 93,179	$ 97,225