Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Service cost	$ 319	$ 1,310	$ 1,323
Interest cost	537	453	383
Net periodic benefit (gain) cost	$ 856	$ 1,763	$ 1,706