Investment Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Amortized Cost	$ 20,378	$ 20,428
Gross Unrealized Gains	193	321
Gross Unrealized Losses	863	731
Estimated Fair Value	19,708	20,018
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Investments [Line Items]
Amortized Cost	5,132	5,187
Gross Unrealized Gains	181	308
Gross Unrealized Losses	0	0
Estimated Fair Value	5,313	5,495
State and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Amortized Cost	5,798	5,797
Gross Unrealized Gains	9	10
Gross Unrealized Losses	0	0
Estimated Fair Value	5,807	5,807
Residential Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	21	23
Gross Unrealized Gains	3	3
Gross Unrealized Losses	0	0
Estimated Fair Value	24	26
Single Issue Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	9,407	9,401
Gross Unrealized Gains	0	0
Gross Unrealized Losses	863	731
Estimated Fair Value	8,544	8,670
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost	20	20
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 20	$ 20