Benefit Plans - Defined Benefit Plans - Estimated Net Actuarial Loss and Prior Service Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Supplemental Retirement Plan
Estimated net actuarial loss and prior service cost that will be amortized from accumulated other comprehensive loss into net periodic cost over next fiscal year
Estimated net actuarial loss and prior service cost that will be amortized from accumulated other comprehensive loss into net periodic cost over next fiscal year	$ 292	$ 276
Split-Dollar Life Insurance Benefit Plan
Estimated net actuarial loss and prior service cost that will be amortized from accumulated other comprehensive loss into net periodic cost over next fiscal year
Estimated net actuarial loss and prior service cost that will be amortized from accumulated other comprehensive loss into net periodic cost over next fiscal year	$ 90	$ 90