Pension Plan and Employee Benefits - Benefit Obligation, Plan Assets, and Funded Status (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 12, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
PENSION BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period		$ 530,936	$ 567,215
Service cost		8,414	9,507	$ 9,039
Interest cost		22,485	20,860	21,648
Plan participants’ contributions		0	0
Actuarial loss (gain)		73,655	(42,935)
Expenses paid		(2,933)	(2,786)
Benefits paid		(22,234)	(20,925)
Benefit obligation at end of period		610,323	530,936	567,215
Change in plan assets
Fair value of plan assets at beginning of period		391,933	444,089
Actual return on plan assets		81,081	(28,884)
Employer contributions	$ 12,300	12,250	0
Expenses paid		(2,933)	(2,786)
Adjustment		0	439
Benefits paid		(22,234)	(20,925)
Fair value of plan assets at end of period		460,097	391,933	444,089
Unfunded status		(150,226)	(139,003)
Accumulated benefit obligation		568,354	491,522
OTHER BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period		40,455	43,203
Service cost		1,191	1,320	1,446
Interest cost		1,646	1,465	1,569
Plan participants’ contributions		1,229	1,224
Actuarial loss (gain)		13,897	(1,106)
Expenses paid		0	0
Benefits paid		(5,696)	(5,651)
Benefit obligation at end of period		52,722	40,455	43,203
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
Adjustment		0	0
Benefits paid		0	0
Fair value of plan assets at end of period		0	0	0
Unfunded status		(52,722)	(40,455)
SERP BENEFITS
Change in benefit obligation
Benefit obligation at beginning of period		78,414	84,339
Service cost		330	542	494
Interest cost		3,326	3,077	3,239
Actuarial loss (gain)		11,608	(5,163)
Benefits paid		(4,550)	(4,381)
Benefit obligation at end of period		89,128	78,414	$ 84,339
Change in plan assets
Accumulated benefit obligation		$ 89,128	$ 78,414