Employee Benefit Plans - Net periodic benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 270	$ 305
Interest cost	5,151	5,712
Recognized actuarial loss	993	1,429
Expected return on plan assets	(6,210)	(6,213)
Net periodic benefit cost	$ 204	$ 1,233