Investment Securities - Summary of Maturities of Debt Securities Held to Maturity by Amortized Cost and Estimated Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 0
Due after one year through five years, Amortized Cost	9,344
Due after five years through ten years, Amortized Cost	5,663
Due after ten years, Amortized Cost	5,421
Amortized Cost	20,428	$ 33,258
Due in one year or less, Estimated Fair Value	0
Due after one year through five years, Estimated Fair Value	9,660
Due after five years through ten year, Estimated Fair Value	5,343
Due after ten years, Estimated Fair Value	5,015
Estimated Fair Value, Total	$ 20,018	$ 31,178