Employee Benefit Plans - Change in projected benefit obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, January 1	$ 655,822	$ 621,312
Service cost	4,947	137
Interest cost	26,583	27,751
Actuarial (gain) loss	37,752	28,247
Regular benefits paid	(23,774)	(21,625)
Curtailments	(17,244)	0
Amendment	1,876	0
Benefit obligation, December 31	685,962	655,822	$ 621,312
Accumulated benefit obligation	685,962	637,153
Defined Benefit Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, January 1	592,290	560,817
Service cost	3,506	(1,403)	12,851
Interest cost	24,205	25,263	23,987
Actuarial (gain) loss	40,840	24,928
Regular benefits paid	(19,737)	(17,315)
Curtailments	(17,244)	0
Amendment	1,852	0
Benefit obligation, December 31	625,712	592,290	560,817
Accumulated benefit obligation	625,712	575,024
Post-Retirement Medical Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, January 1	19,031	16,637
Service cost	1,457	1,246	1,042
Interest cost	758	713	560
Actuarial (gain) loss	(1,216)	1,408
Regular benefits paid	(701)	(973)
Curtailments	0	0
Amendment	0	0
Benefit obligation, December 31	19,329	19,031	16,637
Accumulated benefit obligation	19,329	19,031
Supplemental Executive Retirement Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, January 1	44,501	43,858
Service cost	(16)	294	282
Interest cost	1,620	1,775	2,122
Actuarial (gain) loss	(1,872)	1,911
Regular benefits paid	(3,336)	(3,337)
Curtailments	0	0
Amendment	24	0
Benefit obligation, December 31	40,921	44,501	$ 43,858
Accumulated benefit obligation	$ 40,921	$ 43,098