Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Components of Net Periodic Benefit Costs for Both Pension Plans
Service cost	$ 2,050	$ 1,987
Interest cost	727	624
Expected return on plan assets that reduces periodic pension costs	(559)	(545)
Amortization of prior service costs	4	4
Amortization of net actuarial loss	396	372
Net periodic benefit cost	$ 2,618	$ 2,442