Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,150	$ 2,528
Interest cost	4,960	4,615
Expected return on plan assets	(6,177)	(5,703)
Amortizations
Net loss	3,085	4,763
Total benefit cost	4,018	6,203
OTHER BENEFITS
Components of periodic benefit costs
Service cost	551	595
Interest cost	371	323
Expected return on plan assets	0	0
Amortizations
Net loss	303	384
Total benefit cost	1,225	1,302
SERP
Components of periodic benefit costs
Service cost	57	55
Interest cost	670	625
Amortizations
Prior period service credit	(54)	(53)
Net loss	262	1,016
Total benefit cost	$ 935	$ 1,643