Pension plan and 401(k) Program - Net Periodic Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Retirement Benefits [Abstract]
Service cost	$ 632	$ 557
Interest cost	61	51
Expected return on plan assets	(175)	(189)
Amortization of:
Prior service cost (benefit)	(5)	(5)
Net actuarial loss	49	0
Participant contributions	(194)	(168)
Net periodic benefit cost	$ 368	$ 246