Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of Net Periodic Benefit Costs for the Pension Plans
Service cost	$ 1,660	$ 2,050
Interest cost	673	727
Expected return on plan assets that reduces periodic pension benefit cost	(561)	(559)
Amortization of prior service costs	4	4
Amortization of net actuarial loss	324	396
Net periodic benefit cost	$ 2,100	$ 2,618