Employee Benefit Plans - Amount in AOCI not yet realized (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), gross	$ (114,383)
Net prior service (cost) credit, Gross	(1,052)
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Gross	(115,435)
Net gain (loss), Net of tax	(90,361)
Net prior service (cost) credit, net of tax	(831)
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Net of tax	(91,192)
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), gross	(121,645)
Net prior service (cost) credit, Gross	0
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Gross	(121,645)
Net gain (loss), Net of tax	(96,098)
Net prior service (cost) credit, net of tax	0
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Net of tax	(96,098)
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), gross	6,105
Net prior service (cost) credit, Gross	171
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Gross	6,276
Net gain (loss), Net of tax	4,823
Net prior service (cost) credit, net of tax	135
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Net of tax	4,958
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss), gross	1,157
Net prior service (cost) credit, Gross	(1,223)
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Gross	(66)
Net gain (loss), Net of tax	914
Net prior service (cost) credit, net of tax	(966)
Amounts in accumulated other comprehensive income (loss) that have not yet been recognized as components of net periodic benefit cost, Net of tax	$ (52)