Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of Net Periodic Benefit Costs for the Pension Plans
Service cost	$ 1,660	$ 2,050	$ 4,979	$ 6,150
Interest cost	673	727	2,017	2,181
Expected return on plan assets that reduces periodic pension benefit cost	(561)	(559)	(1,683)	(1,677)
Amortization of prior service costs	4	4	13	13
Amortization of net actuarial loss	324	396	973	1,187
Net periodic benefit cost	$ 2,100	$ 2,618	$ 6,299	$ 7,854