Employee Benefit Plans - Underfunded status (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	$ (202,320)	$ (202,735)
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(142,070)	(139,203)
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(19,329)	(19,031)
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	$ (40,921)	$ (44,501)