Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	$ 270,561	$ 343,284
Loans held for sale, at fair value	83,541	62,593
Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans held for sale, at fair value	83,541	62,593
Purchased certificate of deposit option	809	882
Written certificate of deposit option	809	882
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Government Sponsored Entities' Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	123,817	186,033
Fair Value, Measurements, Recurring [Member] | States of the U.S. and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	59,623	57,757
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed GSE Securities: Residential [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	87,121	99,494
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans held for sale, at fair value	18,525	8,832
Purchased certificate of deposit option	809	882
Written certificate of deposit option	809	882
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury and Government Sponsored Entities' Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	123,817	186,033
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | States of the U.S. and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	59,623	57,757
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage-Backed GSE Securities: Residential [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities, Fair value	87,121	99,494
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans held for sale, at fair value	$ 65,016	$ 53,761