Pension Plan and Employee Benefits - Components of Periodic Benefit Costs and Weighted-Average Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted-average assumptions used to determine the benefit obligation
Rate of compensation increase	2.75%	2.81%
Weighted-average assumptions used to determine the net benefit cost
Rate of compensation increase	2.75%	2.81%	2.93%
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 9,820	$ 8,414	$ 9,507
Interest cost	20,816	22,485	20,860
Expected return on plan assets	(24,974)	(26,502)	(23,773)
Amortizations
Prior service credit	(60)	(71)	(71)
Net loss (gain)	16,292	7,849	12,312
Net periodic benefit cost	$ 21,894	$ 12,175	$ 18,835
Weighted-average assumptions used to determine the benefit obligation
Discount rate	2.74%	3.43%
Weighted-average assumptions used to determine the net benefit cost
Discount rate	3.43%	4.35%	3.73%
Expected return on plan assets	5.91%	6.55%	5.86%
OTHER BENEFITS
Components of periodic benefit costs
Service cost	$ 2,153	$ 1,191	$ 1,320
Interest cost	1,651	1,646	1,465
Expected return on plan assets	0	0	0
Amortizations
Prior service credit	0	0	0
Net loss (gain)	1,389	21	135
Net periodic benefit cost	$ 5,193	$ 2,858	$ 2,920
Weighted-average assumptions used to determine the benefit obligation
Discount rate	2.39%	3.25%
Weighted-average assumptions used to determine the net benefit cost
Discount rate	3.25%	4.16%	3.47%
SERP BENEFITS
Components of periodic benefit costs
Service cost	$ 399	$ 330	$ 542
Interest cost	2,932	3,326	3,077
Amortizations
Prior service credit	(215)	(160)	(160)
Net loss (gain)	3,186	1,544	2,913
Net periodic benefit cost	$ 6,302	$ 5,040	$ 6,372
Weighted-average assumptions used to determine the benefit obligation
Discount rate	2.64%	3.37%
Rate of compensation increase		5.00%
Weighted-average assumptions used to determine the net benefit cost
Discount rate	3.37%	4.34%	3.70%
Rate of compensation increase		5.00%	5.00%