Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,455	$ 2,104	$ 7,365	$ 6,311
Interest cost	5,204	5,621	15,612	16,864
Expected return on plan assets	(6,244)	(6,625)	(18,731)	(19,876)
Amortizations
Prior period service credit	(15)	(18)	(45)	(54)
Net loss	4,073	1,962	12,222	5,887
Net periodic benefit cost	5,473	3,044	16,423	9,132
OTHER BENEFITS
Components of periodic benefit costs
Service cost	600	328	1,615	896
Interest cost	418	435	1,238	1,235
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss	362	105	1,042	25
Net periodic benefit cost	1,380	868	3,895	2,156
SERP
Components of periodic benefit costs
Service cost	100	82	300	247
Interest cost	733	831	2,199	2,494
Amortizations
Prior period service credit	(54)	(34)	(161)	(104)
Net loss	796	381	2,388	1,143
Net periodic benefit cost	$ 1,575	$ 1,260	$ 4,726	$ 3,780