Employee Benefit Plans - Amounts in AOCI expected to be realized (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), Gross	$ (2,015)
Net prior service (cost) credit, Gross	(304)
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Gross	(2,319)
Net gain (loss), Net of tax	(1,591)
Net prior service (cost) credit, Net of tax	(240)
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Net of tax	(1,831)
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), Gross	(2,336)
Net prior service (cost) credit, Gross	0
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Gross	(2,336)
Net gain (loss), Net of tax	(1,845)
Net prior service (cost) credit, Net of tax	0
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Net of tax	(1,845)
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), Gross	276
Net prior service (cost) credit, Gross	20
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Gross	296
Net gain (loss), Net of tax	218
Net prior service (cost) credit, Net of tax	16
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Net of tax	234
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), Gross	45
Net prior service (cost) credit, Gross	(324)
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Gross	(279)
Net gain (loss), Net of tax	36
Net prior service (cost) credit, Net of tax	(256)
Amounts in accumulated other comprehensive income (loss) that are expected to be recognized as components of net periodic benefit costs during the next fiscal year, Net of tax	$ (220)