Employee Benefit Plans - Net Periodic Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic cost (benefit):
Service cost	$ 376	$ 1,686	$ 752	$ (28)
Interest cost	6,220	6,714	12,441	13,428
Expected return on plan assets	(5,331)	(5,118)	(10,661)	(10,236)
Amortization of unrecognized prior service costs (benefits)	76	73	152	146
Amortization of losses (gains) from earlier periods	503	2,073	1,007	4,146
Net periodic cost (benefit)	1,844	5,428	3,691	7,456
Defined Benefit Pension Plan
Components of net periodic cost (benefit):
Service cost	0	1,333	0	(734)
Interest cost	5,709	6,121	11,419	12,242
Expected return on plan assets	(5,331)	(5,118)	(10,661)	(10,236)
Amortization of unrecognized prior service costs (benefits)	0	0	0	0
Amortization of losses (gains) from earlier periods	584	2,199	1,168	4,398
Net periodic cost (benefit)	962	4,535	1,926	5,670
Post-Retirement Medical Plan
Components of net periodic cost (benefit):
Service cost	376	357	752	714
Interest cost	172	188	344	376
Expected return on plan assets	0	0	0	0
Amortization of unrecognized prior service costs (benefits)	(5)	(52)	(10)	(104)
Amortization of losses (gains) from earlier periods	(70)	(113)	(138)	(226)
Net periodic cost (benefit)	473	380	948	760
Payments made	34	207	117	376
Supplemental Executive Retirement Plan
Components of net periodic cost (benefit):
Service cost	0	(4)	0	(8)
Interest cost	339	405	678	810
Expected return on plan assets	0	0	0	0
Amortization of unrecognized prior service costs (benefits)	81	125	162	250
Amortization of losses (gains) from earlier periods	(11)	(13)	(23)	(26)
Net periodic cost (benefit)	409	513	817	1,026
Payments made	$ 599	$ 834	$ 1,199	$ 1,668