Pension Plan and 401(k) Program - Net Periodic Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Retirement Benefits [Abstract]
Service cost	$ 839	$ 629	$ 1,675	$ 1,261
Interest cost	23	61	45	122
Expected return on plan assets	(167)	(174)	(337)	(349)
Amortization of:
Prior service (benefit)	(5)	(5)	(10)	(10)
Net actuarial loss	141	48	287	97
Participant contributions	(214)	(193)	(427)	(387)
Net periodic benefit cost	$ 617	$ 366	$ 1,233	$ 734