Employee Retirement and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plans
Components of net periodic benefit cost:
Service cost	$ 672	$ 732
Interest cost	3,084	2,828
Expected return on plan assets	(3,996)	(4,411)
Amortizations:
Prior service cost	108	108
Unrecognized net (gain) loss	2,440	2,130
Net periodic benefit cost	2,308	1,387
Postretirement Benefits
Components of net periodic benefit cost:
Service cost	154	170
Interest cost	281	235
Amortizations:
Prior service cost	23	23
Unrecognized net (gain) loss	(151)	(118)
Net periodic benefit cost	$ 307	$ 310