Employee Benefits (Details) - Components of Net Periodic Pension Expense - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	$ 2,820	$ 2,780	$ 2,620
Interest cost	765	850	790
Expected return on plan assets	(360)	(375)	(345)
Recognition of actuarial loss	350	200	300
Recognition of termination benefits	0	0	85
Total defined benefit pension expense	$ 3,575	$ 3,455	$ 3,450