Employee Benefit Plans - Change in plan assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Value of plan assets, January 1	$ 453,087	$ 427,131
Actual return on plan assets	50,292	43,271
Employer contributions	4,038	4,310
Benefits paid	(23,775)	(21,625)
Value of plan assets, December 31	483,642	453,087
Defined Benefit Pension Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Value of plan assets, January 1	453,087	427,131
Actual return on plan assets	50,292	43,271
Employer contributions	0	0
Benefits paid	(19,737)	(17,315)
Value of plan assets, December 31	483,642	453,087
Post-Retirement Medical Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	701	973
Benefits paid	(701)	(973)
Supplemental Executive Retirement Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions	3,337	3,337
Benefits paid	$ (3,337)	$ (3,337)