Employee Retirement and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans
Components of net periodic benefit cost:
Service cost	$ 732	$ 752	$ 2,196	$ 2,256
Interest cost	2,828	2,927	8,484	8,781
Expected return on plan assets	(4,411)	(4,758)	(13,233)	(14,274)
Amortizations:
Prior service cost	108	176	324	528
Unrecognized net (gain) loss	2,130	2,581	6,390	7,743
Net periodic benefit cost	1,387	1,678	4,161	5,034
Postretirement Benefits
Components of net periodic benefit cost:
Service cost	170	146	510	438
Interest cost	235	240	705	720
Amortizations:
Prior service cost	23	23	69	69
Unrecognized net (gain) loss	(118)	(114)	(354)	(342)
Net periodic benefit cost	$ 310	$ 295	$ 930	$ 885