Pension and Other Post-Retirement Benefits - Benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Retirement Benefits [Abstract]
Service costs	$ 226	$ 220	$ 444	$ 441
Interest cost	79	80	157	158
Expected return on plan assets	(40)	(40)	(79)	(80)
Amortization of actuarial gains	14	(20)	(52)	(38)
Net periodic benefit cost	$ 279	$ 240	$ 470	$ 481