Pension Plan and Employee Benefits - Net Periodic Pension and Benefits Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
PENSION BENEFITS
Components of periodic benefit costs
Service cost	$ 2,582	$ 2,140	$ 4,910	$ 4,207
Interest cost	5,278	5,593	10,408	11,243
Expected return on plan assets	(6,242)	(6,629)	(12,487)	(13,251)
Amortizations
Prior period service credit	(15)	(18)	(30)	(36)
Net loss (gain)	4,477	2,050	8,149	3,925
Net periodic benefit cost	6,080	3,136	10,950	6,088
OTHER BENEFITS
Components of periodic benefit costs
Service cost	508	280	1,015	568
Interest cost	410	400	820	800
Expected return on plan assets	0	0	0	0
Amortizations
Prior period service credit	0	0	0	0
Net loss (gain)	340	(35)	680	(80)
Net periodic benefit cost	1,258	645	2,515	1,288
SERP
Components of periodic benefit costs
Service cost	105	52	200	165
Interest cost	734	838	1,466	1,663
Amortizations
Prior period service credit	(67)	(35)	(107)	(70)
Net loss (gain)	835	370	1,592	762
Net periodic benefit cost	$ 1,607	$ 1,225	$ 3,151	$ 2,520