Employee Benefit Plans - Target allocation of plan assets (Details) - Defined Benefit Pension Plan	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	100.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	30.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	52.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	18.00%