Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available for Sale, Amortized Cost
Due in one year or less	$ 14,138
Due after one year through five years	73,990
Due after five years through ten years	45,274
Due after ten years	128,406
Total	261,808
Total securities	767,558	$ 752,260
Available for Sale, Fair Value
Due in one year or less	14,207
Due after one year through five years	76,199
Due after five years through ten years	46,617
Due after ten years	128,770
Total	265,793
Total securities	783,331	755,900
Held to Maturity, Amortized Cost
Due in one year or less	8,697
Due after one year through five years	21,870
Due after five years through ten years	5,872
Due after ten years	0
Total	36,439
Total securities	97,893	123,975
Held to Maturity, Fair Value
Due in one year or less	8,791
Due after one year through five years	22,847
Due after five years through ten years	5,927
Due after ten years	0
Total	37,565
Total securities	101,205	$ 124,904
Mortgage-backed and asset-backed securities
Available for Sale, Amortized Cost
Mortgage-backed and asset-backed securities	505,750
Available for Sale, Fair Value
Mortgage-backed and asset-backed securities	517,538
Held to Maturity, Amortized Cost
Mortgage-backed and asset-backed securities	61,454
Held to Maturity, Fair Value
Mortgage-backed and asset-backed securities	$ 63,640