Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of Net Periodic Benefit Costs for the Pension Plans
Service cost	$ 1,705	$ 1,269	$ 3,365	$ 3,319
Interest cost	637	617	1,310	1,344
Expected return on plan assets that reduces periodic pension benefit cost	(370)	(563)	(931)	(1,122)
Amortization of prior service cost	5	5	9	9
Amortization of net actuarial loss	340	253	664	649
Net periodic benefit cost	$ 2,317	$ 1,581	$ 4,417	$ 4,199