Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	$ 808	$ 891	$ 2,422	$ 2,706
Expected return on plan assets	(1,087)	(1,051)	(3,259)	(3,098)
Amortization of net actuarial loss	259	282	778	861
Net settlement loss	1,004	1,218	1,004	1,218
Total net periodic benefit cost (credit)	984	1,340	945	1,687
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Interest cost	24	27	72	81
Amortization of net actuarial loss	64	61	193	184
Amortization of prior service credit	(329)	(328)	(986)	(985)
Total net periodic benefit cost (credit)	$ (241)	$ (240)	$ (721)	$ (720)