Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,067	$ 2,393
Interest cost	5,650	5,183
Expected return on plan assets	(6,622)	(5,938)
Amortizations
Prior period service credit	(18)	(18)
Net loss (gain)	1,875	2,960
Net periodic benefit cost	2,952	4,580
OTHER BENEFITS
Components of periodic benefit costs
Service cost	288	338
Interest cost	400	357
Expected return on plan assets	0	0
Amortizations
Prior period service credit	0	0
Net loss (gain)	(45)	5
Net periodic benefit cost	643	700
SERP
Components of periodic benefit costs
Service cost	113	105
Interest cost	825	760
Amortizations
Prior period service credit	(35)	(35)
Net loss (gain)	392	585
Net periodic benefit cost	$ 1,295	$ 1,415