Employee Benefit Plans - Amount recognized in consolidated balance sheet (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)
Other liabilities
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(202,320)	(202,735)
Other liabilities | Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(142,070)	(139,203)
Other liabilities | Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(19,329)	(19,031)
Other liabilities | Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(40,921)	(44,501)
Accumulated other comprehensive income (loss)
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(115,435)	(129,700)
Accumulated other comprehensive income (loss) | Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	(121,645)	(133,055)
Accumulated other comprehensive income (loss) | Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	6,276	5,715
Accumulated other comprehensive income (loss) | Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Under funded status at December 31	$ (66)	$ (2,360)