Employee Retirement and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plans
Components of net periodic benefit cost:
Service cost	$ 732	$ 752	$ 1,464	$ 1,504
Interest cost	2,828	2,927	5,656	5,854
Expected return on plan assets	(4,411)	(4,758)	(8,822)	(9,516)
Amortizations:
Prior service cost	108	176	216	352
Unrecognized net (gain) loss	2,130	2,581	4,260	5,162
Net periodic benefit cost	1,387	1,678	2,774	3,356
Postretirement Benefits
Components of net periodic benefit cost:
Service cost	170	146	340	292
Interest cost	235	240	470	480
Amortizations:
Prior service cost	23	23	46	46
Unrecognized net (gain) loss	(118)	(114)	(236)	(228)
Net periodic benefit cost	$ 310	$ 295	$ 620	$ 590