Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,140	$ 2,361	$ 4,207	$ 4,753
Interest cost	5,593	5,472	11,243	10,654
Expected return on plan assets	(6,629)	(5,938)	(13,251)	(11,875)
Amortizations
Prior period service credit	(18)	(18)	(36)	(36)
Net loss (gain)	2,050	2,960	3,925	5,921
Net periodic benefit cost	3,136	4,837	6,088	9,417
OTHER BENEFITS
Components of periodic benefit costs
Service cost	280	338	568	675
Interest cost	400	357	800	715
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss (gain)	(35)	5	(80)	10
Net periodic benefit cost	645	700	1,288	1,400
SERP
Components of periodic benefit costs
Service cost	52	166	165	271
Interest cost	838	1,004	1,663	1,764
Amortizations
Prior period service credit	(35)	(47)	(70)	(81)
Net loss (gain)	370	647	762	1,232
Net periodic benefit cost	$ 1,225	$ 1,770	$ 2,520	$ 3,186