Employee Benefit Plans - Net periodic benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 267	$ 258
Interest cost	4,151	4,228
Recognized actuarial loss	2,625	1,325
Expected return on plan assets	(7,339)	(6,506)
Net periodic benefit cost	$ (296)	$ (695)