Securities - Proceeds from Sales and Associated Gains and Losses Reclassified from Other comprehensive Income to Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Proceeds	$ 84,087	$ 70,957	$ 17,105
Gross gains	271	893	370
Income tax expense on net realized gains	$ 103	$ 342	$ 142