Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Components of Net Periodic Benefit Costs for the Pension Plans
Service cost	$ 1,683	$ 1,660
Interest cost	656	673
Expected return on plan assets that reduces periodic pension benefit cost	(466)	(561)
Amortization of prior service cost	4	4
Amortization of net actuarial loss	332	324
Net periodic benefit cost	$ 2,209	$ 2,100