Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Components of Net Periodic Benefit Costs for Both Pension Plans
Service cost	$ 2,390	$ 1,595	$ 3,974	$ 3,168
Interest cost	700	688	1,248	1,095
Expected return on plan assets that reduces periodic pension costs	(597)	(604)	(1,091)	(989)
Amortization of prior service costs	5	5	9	9
Amortization of net actuarial loss	571	38	743	344
Net periodic benefit cost	$ 3,069	$ 1,722	$ 4,883	$ 3,627