Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,629	$ 2,455	$ 7,887	$ 7,365
Interest cost	4,667	5,204	14,001	15,612
Expected return on plan assets	(5,700)	(6,244)	(17,101)	(18,731)
Amortizations
Prior period service credit	0	(15)	0	(45)
Net loss	5,184	4,073	15,553	12,222
Net periodic benefit cost	6,780	5,473	20,340	16,423
Special/contractual termination benefits	3,270	0	3,270	0
Total benefit cost	10,050	5,473	23,610	16,423
OTHER BENEFITS
Components of periodic benefit costs
Service cost	629	600	1,819	1,615
Interest cost	317	418	962	1,238
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss	373	362	1,143	1,042
Net periodic benefit cost	1,319	1,380	3,924	3,895
Special/contractual termination benefits	0	0	0	0
Total benefit cost	1,319	1,380	3,924	3,895
SERP
Components of periodic benefit costs
Service cost	59	100	174	300
Interest cost	634	733	1,903	2,199
Amortizations
Prior period service credit	(54)	(54)	(161)	(161)
Net loss	307	796	921	2,388
Total benefit cost	$ 946	$ 1,575	$ 2,837	$ 4,726