Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available for Sale, Amortized Cost
Due in one year or less	$ 21,258
Due after one year through five years	114,026
Due after five years through ten years	41,818
Due after ten years	27,655
Total	204,757
Available for Sale, Fair Value
Due in one year or less	21,170
Due after one year through five years	111,549
Due after five years through ten years	40,686
Due after ten years	27,237
Total	200,642
Held to Maturity, Amortized Cost
Due in one year or less	13,063
Due after one year through five years	41,523
Due after five years through ten years	25,533
Due after ten years	3,270
Total	83,389
Held to Maturity, Fair Value
Due in one year or less	13,074
Due after one year through five years	40,979
Due after five years through ten years	24,926
Due after ten years	3,178
Total	82,157
Available-for-sale Securities, Amortized Cost Basis	622,818
Available-for-sale Securities	606,231
Held-to-maturity Securities	158,832	$ 139,685
Held-to-maturity Securities, Fair Value	155,316	$ 138,688
Mortgage-backed securities
Available for Sale, Amortized Cost
Available for Sale, Amortized Cost, Without single maturity date	418,061
Available for Sale, Fair Value
Available for Sale, Fair Value, Without single maturity date	405,589
Held to Maturity, Amortized Cost
Held to Maturity, Amortized Cost, Without single maturity date	75,443
Held to Maturity, Fair Value
Held to Maturity, Fair Value, Without single maturity date	$ 73,159