Employee Benefit Plans - Summary of pre tax amounts recognized in comprehensive loss (Details 3) - Retirement Income Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net (gain) loss	$ (799)	$ 2,039	$ (1,621)
Amortization of net loss	(851)	(799)	(790)
Net transition obligation (asset)	0	0	0
Amount recognized in accumulated other comprehensive loss	$ (1,650)	$ 1,240	$ (2,411)