Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Components of Net Periodic Benefit Costs for Both Pension Plans
Service cost	$ 2,050	$ 1,989	$ 6,150	$ 5,963
Interest cost	727	624	2,181	1,872
Expected return on plan assets that reduces periodic pension costs	(559)	(546)	(1,677)	(1,637)
Amortization of prior service costs	4	4	13	13
Amortization of net actuarial loss	396	371	1,187	1,114
Net periodic benefit cost	$ 2,618	$ 2,442	$ 7,854	$ 7,325