Investment Securities - Summary of Maturities of Debt Securities Held to Maturity by Amortized Cost and Estimated Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 2,000	$ 0
Due after one year through five years, Amortized Cost	7,290	9,344
Due after five years through ten years, Amortized Cost	8,092	5,663
Due after ten years, Amortized Cost	2,996	5,421
Amortized Cost	20,378	20,428
Due in one year or less, Estimated Fair Value	2,000	0
Due after one year through five years, Estimated Fair Value	7,478	9,660
Due after five years through ten year, Estimated Fair Value	7,610	5,343
Due after ten years, Estimated Fair Value	2,620	5,015
Estimated Fair Value, Total	$ 19,708	$ 20,018

Amount of investment in debt security measured at amortized cost (held-to-maturity).

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