Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Gross Losses	$ 0	$ 0
Gross Gains	2,840	652
Held-to-maturity Securities Pledged as Collateral	369,400		$ 338,300
Proceeds from Sale of Available-for-sale Securities	0	$ 0
Available-for-sale securities pledged as collateral	$ 609,900		$ 569,000