Pension - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic benefit cost:
Service cost	$ 9	$ 9	$ 18	$ 18
Interest cost	10	14	20	28
Expected return on plan assets	(17)	(20)	(34)	(40)
Amortization of prior service costs		2		4
Amortization of actuarial loss	22	26	44	52
Total non-service cost	15	22	30	44
Net periodic pension cost	$ 24	$ 31	$ 48	$ 62