Employee Benefit Plans - Summary of Projected Benefit Obligation in Excess of Plan Assets for Defined Benefit Plans (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 97,869	$ 94,732
Fair value of plan assets	60,725	55,630
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 64,128	$ 55,464