Employee Benefit Plans - Net periodic benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 257	$ 256
Interest cost	3,035	4,069
Recognized actuarial loss	2,100	1,546
Expected return on plan assets	(7,405)	(7,511)
Prior service cost	56	55
Net periodic benefit cost	$ (1,957)	$ (1,585)