Employee Benefit Plans - Recognized Net Periodic Benefit Cost and Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Sale Of Subsidiary [Abstract]
Service cost	$ 103	$ 103	$ 96
Interest cost	114	108	90
Net amortization of transition obligation and actuarial loss	192	184	173
Net periodic benefit cost	409	395	359
Net (gain) loss	(808)	282	311
Amortization of loss	(192)	(184)	(173)
Total recognized in other comprehensive income	(1,000)	98	138
Total recognized in net periodic benefit cost and other comprehensive income	$ (591)	$ 493	$ 497