Employee Benefit Plans - Summary of Accumulated 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]
Accumulated benefit obligation in excess of plan assets [Abstract]
Accumulated benefit obligation	$ 93,084	$ 88,733
Fair value of plan assets	60,725	55,630
SERP [Member]
Accumulated benefit obligation in excess of plan assets [Abstract]
Accumulated benefit obligation	$ 56,004	$ 49,520