Pension plan and 401(k) Program - Net Periodic Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Service cost	$ 2,166	$ 1,982	$ 2,087
Interest cost	197	253	238
Expected return on plan assets	(731)	(773)	(741)
Amortization of:
Prior service cost (benefit)	(19)	(19)	0
Net actuarial loss	0	139	212
Participant contributions	(753)	(625)	(598)
Net periodic benefit cost	$ 860	$ 957	$ 1,198