Employee Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Net (losses) gains	$ (22,183)	$ (22,065)
Deferred income tax benefit	1,927	1,927
AOCI	(20,256)	(20,138)
Postretirement
Defined Benefit Plan Disclosure [Line Items]
Net (losses) gains	(157)	(67)
Deferred income tax benefit	6	6
AOCI	$ (151)	$ (61)