Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of Net Periodic Benefit Costs for the Pension Plans
Service cost	$ 1,683	$ 1,660	$ 5,048	$ 4,979
Interest cost	657	673	1,967	2,017
Expected return on plan assets that reduces periodic pension benefit cost	(466)	(561)	(1,397)	(1,683)
Amortization of prior service cost	4	4	13	13
Amortization of net actuarial loss	331	324	995	973
Net periodic benefit cost	$ 2,209	$ 2,100	$ 6,626	$ 6,299