Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Components of accumulated other comprehensive loss	$ 156,605	$ 153,966	$ 121,842	$ 120,061
Unrealized Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Components of accumulated other comprehensive loss	(2,246)	(957)	(109)
Pension Liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Components of accumulated other comprehensive loss	(6,035)	(6,135)	(5,648)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Components of accumulated other comprehensive loss	$ (8,281)	$ (7,092)	$ (5,757)	$ (6,024)