Employee Benefit Plans - Net Periodic Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic cost (benefit):
Service cost	$ 317	$ 2,544	$ 1,069	$ 2,516
Interest cost	6,258	6,711	18,700	20,139
Expected return on plan assets	(5,361)	(4,980)	(16,022)	(15,216)
Amortization of unrecognized prior service costs (benefits)	76	(109)	228	37
Amortization of losses (gains) from earlier periods	570	2,302	1,577	6,448
Net periodic cost (benefit)	1,860	6,468	5,552	13,924
Defined Benefit Pension Plan
Components of net periodic cost (benefit):
Service cost	0	2,170	0	1,436
Interest cost	5,736	6,114	17,155	18,356
Expected return on plan assets	(5,361)	(4,980)	(16,022)	(15,216)
Amortization of unrecognized prior service costs (benefits)	0	0	0	0
Amortization of losses (gains) from earlier periods	625	2,241	1,793	6,639
Net periodic cost (benefit)	1,000	5,545	2,926	11,215
Post-Retirement Medical Plan
Components of net periodic cost (benefit):
Service cost	317	378	1,069	1,092
Interest cost	183	192	527	568
Expected return on plan assets	0	0	0	0
Amortization of unrecognized prior service costs (benefits)	(5)	(235)	(15)	(339)
Amortization of losses (gains) from earlier periods	(44)	75	(182)	(151)
Net periodic cost (benefit)	451	410	1,399	1,170
Payments made	94	155	211	531
Supplemental Executive Retirement Plan
Components of net periodic cost (benefit):
Service cost	0	(4)	0	(12)
Interest cost	339	405	1,018	1,215
Expected return on plan assets	0	0	0	0
Amortization of unrecognized prior service costs (benefits)	81	126	243	376
Amortization of losses (gains) from earlier periods	(11)	(14)	(34)	(40)
Net periodic cost (benefit)	409	513	1,227	1,539
Payments made	$ 590	$ 834	$ 1,789	$ 2,502