Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Components of Net Periodic Benefit Costs for the Pension Plans
Service cost	$ 1,108	$ 1,705	$ 2,791	$ 3,365
Interest cost	739	637	1,395	1,310
Expected return on plan assets that reduces periodic pension benefit cost	(321)	(370)	(787)	(931)
Amortization of prior service cost	5	5	9	9
Amortization of net actuarial loss	147	340	479	664
Net periodic benefit cost	$ 1,678	$ 2,317	$ 3,887	$ 4,417