Employee Benefit Plans - Net periodic benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 305	$ 329
Interest cost	5,712	6,733
Recognized actuarial loss	1,429	1,036
Expected return on plan assets	(6,213)	(6,781)
Net periodic benefit cost	$ 1,233	$ 1,317