Pension and Other Post Retirement Benefit Plans - Amount To Be Amortized From Accumulated Other Comprehensive Income, Next Fiscal Year (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]
Amortization of net loss	$ (830)	$ (1,208)	$ (1,505)
Amortization of prior service cost	(87)	(90)	(61)
Total	$ (917)	$ (1,298)	$ (1,566)