Employee Benefit Plans - Summary of amounts recognized in consolidated balance sheets (Details 1) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Long-term pension liabilities	$ (35,635)	$ (32,864)
Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Funded status of the Retirement Income Plan	(8,347)	(9,570)
SERP liability	(27,288)	(23,294)
Long-term pension liabilities	$ (35,635)	$ (32,864)