Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,328	$ 2,067
Interest cost	5,130	5,650
Expected return on plan assets	(6,245)	(6,622)
Amortizations
Prior period service credit	(15)	(18)
Net loss (gain)	3,672	1,875
Net periodic benefit cost	4,870	2,952
OTHER BENEFITS
Components of periodic benefit costs
Service cost	508	288
Interest cost	410	400
Expected return on plan assets	0	0
Amortizations
Prior period service credit	0	0
Net loss (gain)	339	(45)
Net periodic benefit cost	1,257	643
SERP
Components of periodic benefit costs
Service cost	95	113
Interest cost	733	825
Amortizations
Prior period service credit	(40)	(35)
Net loss (gain)	757	392
Net periodic benefit cost	$ 1,545	$ 1,295