Pension - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of net periodic benefit cost:
Service cost	$ 9	$ 9
Interest cost	10	14
Expected return on plan assets	(17)	(20)
Amortization of prior service costs		2
Amortization of actuarial loss	22	26
Total non-service cost	15	22
Net periodic pension cost	$ 24	$ 31