Employee Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic cost (benefit):
Service cost	$ 1,686	$ 1,701	$ (28)	$ 3,402
Interest cost	6,714	6,901	13,428	13,802
Expected return on plan assets	(5,118)	(6,278)	(10,236)	(12,556)
Amortization of unrecognized prior service costs (benefits)	73	(151)	146	(302)
Amortization of losses (gains) from earlier periods	2,073	2,385	4,146	4,770
Net periodic cost (benefit)	5,428	4,558	7,456	9,116
Defined Benefit Pension Plan
Components of net periodic cost (benefit):
Service cost	1,333	1,335	(734)	2,670
Interest cost	6,121	6,282	12,242	12,564
Expected return on plan assets	(5,118)	(6,278)	(10,236)	(12,556)
Amortization of unrecognized prior service costs (benefits)	0	0	0	0
Amortization of losses (gains) from earlier periods	2,199	2,485	4,398	4,970
Net periodic cost (benefit)	4,535	3,824	5,670	7,648
Post-Retirement Medical Plan
Components of net periodic cost (benefit):
Service cost	357	293	714	586
Interest cost	188	175	376	350
Expected return on plan assets	0	0	0	0
Amortization of unrecognized prior service costs (benefits)	(52)	(276)	(104)	(552)
Amortization of losses (gains) from earlier periods	(113)	(85)	(226)	(170)
Net periodic cost (benefit)	380	107	760	214
Expected Employer obligation for the year	207	187	376	391
Supplemental Executive Retirement Plan
Components of net periodic cost (benefit):
Service cost	(4)	73	(8)	146
Interest cost	405	444	810	888
Expected return on plan assets	0	0	0	0
Amortization of unrecognized prior service costs (benefits)	125	125	250	250
Amortization of losses (gains) from earlier periods	(13)	(15)	(26)	(30)
Net periodic cost (benefit)	513	627	1,026	1,254
Expected Employer obligation for the year	$ 834	$ 834	$ 1,668	$ 1,668