Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of Net Periodic Benefit Costs for Both Pension Plans
Service cost	$ 1,269	$ 2,113	$ 3,319	$ 4,100
Interest cost	617	830	1,344	1,454
Expected return on plan assets that reduces periodic pension costs	(563)	(573)	(1,122)	(1,118)
Amortization of prior service costs	5	5	9	9
Amortization of net actuarial loss	253	419	649	791
Net periodic benefit cost	$ 1,581	$ 2,794	$ 4,199	$ 5,236