Securities - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of securities holdings | Securities	0	0
Shareholders' equity percentage	10.00%	10.00%
Securities	$ 319,575	$ 329,379
Tax provision related to net realized gains	540	352	$ 233
Proceeds from sale of trading securities	1,091	540	2,129
Net realized (losses) gains on trading securities	93	70	$ (211)
Pooled Trust Preferred [Member]
Schedule of Available-for-sale Securities [Line Items]
Pooled trust preferred securities sold	800	0
Net realized (losses) gains on trading securities	$ 1,383	$ 922