Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available for Sale, Amortized Cost
Due in one year or less	$ 22,184
Due after one year through five years	102,466
Due after five years through ten years	49,826
Due after ten years	28,873
Total	203,349
Amortized Cost	637,626	$ 634,928
Available for Sale, Fair Value
Due in one year or less	22,074
Due after one year through five years	100,885
Due after five years through ten years	48,236
Due after ten years	28,586
Total	199,781
Total securities, Available for Sale, Fair Value	623,242	628,046
Held to Maturity, Amortized Cost
Due in one year or less	13,926
Due after one year through five years	39,602
Due after five years through ten years	27,775
Due after ten years	3,279
Total	84,582
Amortized Cost	152,127	139,685
Held to Maturity, Fair Value
Due in one year or less	13,921
Due after one year through five years	39,172
Due after five years through ten years	27,247
Due after ten years	3,192
Total	83,532
Total securities, Held to Maturity, Fair Value	149,213	$ 138,688
Mortgage-backed securities
Available for Sale, Amortized Cost
Available for Sale, Amortized Cost	434,277
Available for Sale, Fair Value
Available for Sale, Fair Value	423,461
Held to Maturity, Amortized Cost
Held to Maturity, Amortized Cost	67,545
Held to Maturity, Fair Value
Held to Maturity, Fair Value	$ 65,681