Securities - Stratification by Contractual Maturity of Securities (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Maturity, Marurity Date, Amortized Cost [Abstract]
Due in one year or less, Amortized Cost	$ 5,163
Due after one year through five years, Amortized Cost	88,106
Due after five years through ten years, Amortized Cost	341,569
Due after ten years, Amortized Cost	262,875
Securities available for sale, Amortized Cost	697,713
Debt Securities, Available-for-sale, Maturity, Maturity Date, Fair Value [Abstract]
Due in one year or less, Fair Value	5,180
Due after one year through five years, Fair Value	89,216
Due after five years through ten years, Fair Value	343,661
Due after ten years, Fair Value	262,895
Securities available for sale, Fair Value Total	700,952
Debt Securities, Held-to-maturity, Maturity Date, Amortized Cost [Abstract]
Due in one year or less, Amortized Cost	5,163
Due after one year through five years, Amortized Cost	88,106
Due after five years through ten years, Amortized Cost	341,569
Due after ten years, Amortized Cost	262,875
Amortized Cost	697,713
Debt Securities, Held-to-maturity, Maturity Date, Fair Value [Abstract]
Due in one year or less, Fair Value	5,180
Due after one year through five years, Fair Value	89,216
Due after five years through ten years, Fair Value	343,661
Due after ten years, Fair Value	262,895
Securities available for sale, Fair Value Total	$ 700,952