Employee Benefit Plans - Fair Value of Plan Assets by Fund Type (Details) - Retirement Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 18,829	$ 16,778	$ 15,774
Large cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,068	5,058
Mid cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,197	1,022
Small cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,777	1,597
International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,060	1,621
Fixed-income funds—U.S. bonds and short term
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7,727	$ 7,480