Pension and Other Post-Retirement Benefits - Benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Service costs	$ 226	$ 220	$ 661	$ 662
Interest cost	79	80	235	238
Expected return on plan assets	(40)	(40)	(118)	(120)
Amortization of actuarial gains	(19)	(20)	(57)	(49)
Net periodic benefit cost	$ 246	$ 240	$ 721	$ 731