Pension and Other Post-Retirement Benefits - Benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Retirement Benefits [Abstract]
Service costs	$ 368	$ 226	$ 1,104	$ 661
Interest cost	82	79	246	235
Expected return on plan assets	(52)	(40)	(156)	(118)
Amortization of actuarial gains	(4)	(19)	(12)	(57)
Net periodic benefit cost	$ 394	$ 246	$ 1,182	$ 721