Pension Plan and Employee Benefits - Net Periodic Benefit Cost Related to SERP (Details) - SERP Benefits - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of periodic benefit costs
Service cost		$ 136	$ 123	$ 407	$ 370
Interest cost		769	810	2,296	2,430
Amortizations
Prior period service credit		(35)	(57)	(104)	(124)
Net loss		723	536	2,180	1,560
Net periodic benefit cost		1,593	1,412	4,779	4,236
Special/contractual termination benefits	$ 300	0	0	0	315
Net periodic benefit cost		$ 1,593	$ 1,412	$ 4,779	$ 4,551