Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available for Sale, Amortized Cost
Due in one year or less	$ 29,607
Due after one year through five years	103,697
Due after five years through ten years	43,357
Due after ten years	17,653
Total	194,314
Total securities	715,656	$ 649,741
Available for Sale, Fair Value
Due in one year or less	29,610
Due after one year through five years	104,533
Due after five years through ten years	44,014
Due after ten years	17,350
Total	195,507
Total securities	720,405	638,618
Held to Maturity, Amortized Cost
Due in one year or less	7,751
Due after one year through five years	34,577
Due after five years through ten years	24,746
Due after ten years	763
Total	67,837
Investment securities held to maturity; fair value of $152,495 at September 30, 2019 and $150,932 at December 31, 2018	151,115	153,646
Held to Maturity, Fair Value
Due in one year or less	7,751
Due after one year through five years	34,959
Due after five years through ten years	25,057
Due after ten years	765
Total	68,532
Total securities	152,495	$ 150,932
Mortgage-backed securities
Available for Sale, Amortized Cost
Mortgage-backed securities	521,342
Available for Sale, Fair Value
Mortgage-backed securities	524,898
Held to Maturity, Amortized Cost
Mortgage-backed securities	83,278
Held to Maturity, Fair Value
Mortgage-backed securities	$ 83,963