Employee Benefit Plans (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017
Components of net periodic cost (benefit):
Service cost	$ (1,714,000)	$ 1,701,000
Interest cost	6,714,000	6,901,000
Expected return on plan assets	(5,118,000)	(6,278,000)
Amortization of unrecognized prior service costs (benefits)	73,000	(151,000)
Amortization of losses (gains) from earlier periods	2,073,000	2,385,000
Net periodic cost (benefit)	2,028,000	4,558,000
Defined Benefit Pension Plan
Components of net periodic cost (benefit):
Service cost	(2,067,000)	1,335,000
Interest cost	6,121,000	6,282,000
Expected return on plan assets	(5,118,000)	(6,278,000)
Amortization of unrecognized prior service costs (benefits)	0	0
Amortization of losses (gains) from earlier periods	2,199,000	2,485,000
Net periodic cost (benefit)	1,135,000	3,824,000
Post-Retirement Medical Plan
Components of net periodic cost (benefit):
Service cost	357,000	293,000
Interest cost	188,000	175,000
Expected return on plan assets	0	0
Amortization of unrecognized prior service costs (benefits)	(52,000)	(276,000)
Amortization of losses (gains) from earlier periods	(113,000)	(85,000)
Net periodic cost (benefit)	380,000	107,000
Payments to plan	169,000	204,000
Supplemental Executive Retirement Plan
Components of net periodic cost (benefit):
Service cost	(4,000)	73,000
Interest cost	405,000	444,000
Expected return on plan assets	0	0
Amortization of unrecognized prior service costs (benefits)	125,000	125,000
Amortization of losses (gains) from earlier periods	(13,000)	(15,000)
Net periodic cost (benefit)	513,000	627,000
Payments to plan	$ 834,000	$ 834,000
Scenario, Forecast | Defined Benefit Pension Plan
Components of net periodic cost (benefit):
Expected Employer contribution for the year			$ 0
Scenario, Forecast | Post-Retirement Medical Plan
Components of net periodic cost (benefit):
Expected Employer contribution for the year			678,000
Scenario, Forecast | Supplemental Executive Retirement Plan
Components of net periodic cost (benefit):
Expected Employer contribution for the year			$ 3,336,000