Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,528	$ 2,328
Interest cost	4,615	5,130
Expected return on plan assets	(5,703)	(6,245)
Amortizations
Prior period service cost (credit)	0	(15)
Net loss	4,763	3,672
Net periodic benefit cost	6,203	4,870
OTHER BENEFITS
Components of periodic benefit costs
Service cost	595	508
Interest cost	323	410
Expected return on plan assets	0	0
Amortizations
Prior period service cost (credit)	0	0
Net loss	384	339
Net periodic benefit cost	1,302	1,257
SERP
Components of periodic benefit costs
Service cost	55	95
Interest cost	625	733
Amortizations
Prior period service cost (credit)	(53)	(40)
Net loss	1,016	757
Net periodic benefit cost	$ 1,643	$ 1,545