Employee Benefit Plans - Net benefit costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,947	$ 137
Interest cost	26,583	27,751
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,506	(1,403)	$ 12,851
Interest cost	24,205	25,263	23,987
Expected return on plan assets	(20,626)	(22,339)	(28,345)
Amortization of unrecognized prior service cost	0	0	13
Amortization of loss from earlier periods	7,190	10,260	12,398
Net periodic cost	14,275	11,781	20,904
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,457	1,246	1,042
Interest cost	758	713	560
Amortization of unrecognized prior service cost	(453)	(1,102)	(1,640)
Amortization of loss from earlier periods	(202)	(317)	(511)
Net periodic cost	1,560	540	(549)
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	(16)	294	282
Interest cost	1,620	1,775	2,122
Amortization of unrecognized prior service cost	501	501	933
Amortization of loss from earlier periods	(54)	(61)	663
Net periodic cost	$ 2,051	$ 2,509	$ 4,000