Employee and Director Benefit Plans - Summary of Components of Net Periodic Post-Retirement Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Service cost	$ 153	$ 93	$ 94
Interest cost	135	101	102
Amortization of net gain		(73)	(44)
Amortization of prior service credit	(81)	(81)	(82)
Net periodic post-retirement benefit cost (credit)	$ 207	$ 40	$ 70