Employee Benefit Plans - Net periodic benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 258	$ 270
Interest cost	4,228	5,151
Recognized actuarial loss	1,325	993
Expected return on plan assets	(6,506)	(6,210)
Net periodic benefit cost	$ (695)	$ 204