Employee Benefit Plans - Weighted average interest rate assumptions (Details)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.71%	4.20%
Rate of compensation increase	4.47%	4.47%
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.63%	4.05%
Initial health care cost trend	6.50%	6.75%
Ultimate health care cost trend	5.00%	5.00%
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.43%	3.80%
Rate of compensation increase	4.00%	4.00%