Pension plan and 401(k) Program - Net Periodic Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Retirement Benefits [Abstract]
Service cost	$ 629	$ 352	$ 1,261	$ 909
Interest cost	61	32	122	83
Expected return on plan assets	(174)	(120)	(349)	(309)
Amortization of:
Prior service (benefit)	(5)	(3)	(10)	(8)
Net actuarial loss	48	0	97	0
Participant contributions	(193)	(106)	(387)	(274)
Net periodic benefit cost	$ 366	$ 155	$ 734	$ 401