Pension Plan and Employee Benefits - Benefit Obligation, Plan Assets, and Funded Status (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Dec. 04, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
PENSION BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period			$ 610,323	$ 530,936
Service cost			9,820	8,414	$ 9,507
Interest cost			20,816	22,485	20,860
Plan participants’ contributions			0	0
Actuarial loss (gain)			71,708	73,655
Expenses paid			(2,661)	(2,933)
Benefits paid			(23,622)	(22,234)
Benefit obligation at end of period			686,384	610,323	530,936
Change in plan assets
Fair value of plan assets at beginning of period			460,097	391,933
Actual return on plan assets			66,557	81,081
Employer contributions	$ 15,800	$ 12,300	15,750	12,250
Expenses paid			(2,662)	(2,933)
Benefits paid			(23,622)	(22,234)
Fair value of plan assets at end of period			516,120	460,097	391,933
Unfunded status			(170,264)	(150,226)
Accumulated benefit obligation			636,199	568,354
OTHER BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period			52,722	40,455
Service cost			2,153	1,191	1,320
Interest cost			1,651	1,646	1,465
Plan participants’ contributions			1,289	1,229
Actuarial loss (gain)			4,221	13,897
Expenses paid			0	0
Benefits paid			(5,705)	(5,696)
Benefit obligation at end of period			56,331	52,722	40,455
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
Unfunded status			(56,331)	(52,722)
SERP BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period			89,128	78,414
Service cost			399	330	542
Interest cost			2,932	3,326	3,077
Actuarial loss (gain)			9,621	11,608
Benefits paid			(4,590)	(4,550)
Plan amendments			(265)	0
Benefit obligation at end of period			97,225	89,128	$ 78,414
Change in plan assets
Accumulated benefit obligation			$ 97,225	$ 89,128