Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,104	$ 2,377	$ 6,311	$ 7,130
Interest cost	5,621	5,215	16,864	15,869
Expected return on plan assets	(6,625)	(5,943)	(19,876)	(17,818)
Amortizations
Prior period service credit	(18)	(18)	(54)	(53)
Net loss	1,962	3,078	5,887	8,998
Net periodic benefit cost	3,044	4,709	9,132	14,126
OTHER BENEFITS
Components of periodic benefit costs
Service cost	328	336	896	1,011
Interest cost	435	361	1,235	1,076
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss	105	96	25	105
Net periodic benefit cost	868	793	2,156	2,192
SERP
Components of periodic benefit costs
Service cost	82	136	247	407
Interest cost	831	769	2,494	2,296
Amortizations
Prior period service credit	(34)	(35)	(104)	(104)
Net loss	381	723	1,143	2,180
Net periodic benefit cost	$ 1,260	$ 1,593	$ 3,780	$ 4,779