Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available for Sale, Amortized Cost
Due in one year or less	$ 30,766
Due after one year through five years	104,519
Due after five years through ten years	40,768
Due after ten years	18,393
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis	194,446
Total	649,741
Available for Sale, Fair Value
Due in one year or less	30,614
Due after one year through five years	102,754
Due after five years through ten years	40,108
Due after ten years	18,048
Total	191,524
Investment securities, available for sale, at fair value	638,618
Held to Maturity, Amortized Cost
Due in one year or less	7,061
Due after one year through five years	42,103
Due after five years through ten years	23,752
Due after ten years	3,018
Total	75,934
Amortized Cost	153,646	$ 139,685
Held to Maturity, Fair Value
Due in one year or less	7,066
Due after one year through five years	41,720
Due after five years through ten years	23,185
Due after ten years	2,953
Total	74,924
Total securities, Held to Maturity, Fair Value	150,932	$ 138,688
Mortgage-backed securities
Available for Sale, Amortized Cost
Available for sale	455,295
Available for Sale, Fair Value
Available for sale	447,094
Held to Maturity, Amortized Cost
Held to maturity	77,712
Held to Maturity, Fair Value
Held to maturity	$ 76,008