Pension and Other Post-Retirement Benefits - Benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Service costs	$ 1,136	$ 870	$ 888
Interest cost	292	311	283
Expected return on plan assets	(178)	(157)	(150)
Amortization of actuarial gains	(46)	(76)	(78)
Net periodic benefit cost	705	955	943
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	$ (499)	$ 7	$ 0