Pension and Other Post-Retirement Benefits - Benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Retirement Benefits [Abstract]
Service costs	$ 226	$ 221
Interest cost	79	78
Expected return on plan assets	(40)	(40)
Amortization of actuarial gains	(19)	(18)
Net periodic benefit cost	$ 246	$ 241