Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Components of the net periodic benefit cost
Interest cost	$ 33	$ 39
Recognized net actuarial loss	61	63
Benefit cost (income)	94	102
Postretirement Benefits
Components of the net periodic benefit cost
Service cost	87	123
Interest cost	187	300
Amortization of prior service cost	(228)	0
Benefit cost (income)	$ 46	$ 423