Employee Benefit Plans (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Components of the net periodic benefit cost
Service cost	$ 0	$ 0	$ 236
Interest cost	155	194	182
Expected return on plan assets	0	0	0
Recognized net actuarial loss	252	146	1,516
Benefit cost (income)	407	340	1,934
Postretirement Benefits
Components of the net periodic benefit cost
Service cost	391	760	900
Interest cost	985	1,549	1,584
Expected return on plan assets	(455)	(355)	(543)
Recognized net actuarial loss	0	768	1,517
Benefit cost (income)	$ 921	$ 2,722	$ 3,458