Employee Benefit Plans - Net Periodic Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of net periodic cost (benefit):
Service cost	$ 376	$ (1,714)
Interest cost	6,220	6,714
Expected return on plan assets	(5,331)	(5,118)
Amortization of unrecognized prior service costs (benefits)	12	73
Amortization of losses (gains) from earlier periods	568	2,073
Net periodic cost (benefit)	1,845	2,028
Defined Benefit Pension Plan
Components of net periodic cost (benefit):
Service cost	0	(2,067)
Interest cost	5,709	6,121
Expected return on plan assets	(5,331)	(5,118)
Amortization of unrecognized prior service costs (benefits)	0	0
Amortization of losses (gains) from earlier periods	584	2,199
Net periodic cost (benefit)	962	1,135
Post-Retirement Medical Plan
Components of net periodic cost (benefit):
Service cost	376	357
Interest cost	172	188
Expected return on plan assets	0	0
Amortization of unrecognized prior service costs (benefits)	(69)	(52)
Amortization of losses (gains) from earlier periods	(5)	(113)
Net periodic cost (benefit)	474	380
Payments made	82	169
Supplemental Executive Retirement Plan
Components of net periodic cost (benefit):
Service cost	0	(4)
Interest cost	339	405
Expected return on plan assets	0	0
Amortization of unrecognized prior service costs (benefits)	81	125
Amortization of losses (gains) from earlier periods	(11)	(13)
Net periodic cost (benefit)	409	513
Payments made	$ 599	$ 834