Employee Benefit Plans - Unfunded Post Retirement Benefits Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Sale Of Subsidiary [Abstract]
Benefit obligation at beginning of year	$ 3,409	$ 3,066	$ 2,744
Interest cost	114	108	90
Service cost	103	103	96
Actual claims	(78)	(151)	(109)
Actuarial (gain) loss	(808)	283	245
Benefit obligation at end of year	$ 2,740	$ 3,409	$ 3,066