Investment Securities - Summary of Contractual Maturities of Investment Securities Classified as Available for Sale and Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available for Sale, Amortized Cost
Due in one year or less	$ 27,831
Due after one year through five years	106,416
Due after five years through ten years	37,758
Due after ten years	28,496
Total	200,501
Available for Sale Securities, Amortized Cost Basis	632,646
Available for Sale, Fair Value
Due in one year or less	27,694
Due after one year through five years	103,528
Due after five years through ten years	36,465
Due after ten years	28,009
Total	195,696
Available for Sale Securities	613,243
Held to Maturity, Amortized Cost
Due in one year or less	11,369
Due after one year through five years	42,149
Due after five years through ten years	24,093
Due after ten years	3,261
Total	80,872
Held to Maturity Securities	158,576	$ 139,685
Held to Maturity, Fair Value
Due in one year or less	11,376
Due after one year through five years	41,398
Due after five years through ten years	23,204
Due after ten years	3,137
Total	79,115
Held to Maturity Securities, Fair Value	153,975	$ 138,688
Mortgage-backed securities
Available for Sale, Amortized Cost
Available for Sale, Amortized Cost, Without single maturity date	432,145
Available for Sale, Fair Value
Available for Sale, Fair Value, Without single maturity date	417,547
Held to Maturity, Amortized Cost
Held to Maturity, Amortized Cost, Without single maturity date	77,704
Held to Maturity, Fair Value
Held to Maturity, Fair Value, Without single maturity date	$ 74,860