Pension plan and 401(k) Program - Net Periodic Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Service cost	$ 2,535	$ 2,166	$ 1,982
Interest cost	244	197	253
Expected return on plan assets	(702)	(731)	(773)
Amortization of:
Prior service cost (benefit)	(19)	(19)	(19)
Net actuarial loss	179	0	139
Participant contributions	(832)	(753)	(625)
Net periodic benefit cost	$ 1,405	$ 860	$ 957