Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,730	$ 2,582	$ 5,258	$ 4,910
Interest cost	4,719	5,278	9,334	10,408
Expected return on plan assets	(5,698)	(6,242)	(11,400)	(12,487)
Amortizations
Prior period service credit	0	(15)	0	(30)
Net loss	5,607	4,477	10,369	8,149
Net periodic benefit cost	7,358	6,080	13,561	10,950
OTHER BENEFITS
Components of periodic benefit costs
Service cost	595	508	1,190	1,015
Interest cost	323	410	645	820
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss	384	340	770	680
Net periodic benefit cost	1,302	1,258	2,605	2,515
SERP
Components of periodic benefit costs
Service cost	61	105	116	200
Interest cost	644	734	1,269	1,466
Amortizations
Prior period service credit	(55)	(67)	(107)	(107)
Net loss	(401)	835	613	1,592
Net periodic benefit cost	$ 249	$ 1,607	$ 1,891	$ 3,151