Employee and Director Benefit Plans - Reconciliation of Accumulated Post-Retirement Benefit Obligation, Included Employee and Director Benefit Plans Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Accumulated post-retirement benefit obligation as of December 31, 2016	$ 2,514
Service cost	153	$ 93	$ 94
Interest cost	135	101	$ 102
Actuarial loss	1,078
Benefits paid	(48)
Accumulated post-retirement benefit obligation as of December 31, 2017	$ 3,832	$ 2,514