Fair Value Measurement - Quantitative Information about Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Impaired Originated & PNCI Loans [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 445	$ 2,767
Foreclosed Assets [Member] | Land & Construction [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	27	1,341
Foreclosed Assets [Member] | Residential Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	744	622
Foreclosed Assets [Member] | Commercial Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 92	$ 254
Level 3 [Member] | Sales Comparison Approach [Member] | Minimum [Member] | Impaired Originated & PNCI Loans [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	(55.80%)	(74.00%)
Level 3 [Member] | Sales Comparison Approach [Member] | Minimum [Member] | Foreclosed Assets [Member] | Land & Construction [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average		(53.00%)
Level 3 [Member] | Sales Comparison Approach [Member] | Minimum [Member] | Foreclosed Assets [Member] | Residential Real Estate [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	(47.00%)	(47.00%)
Level 3 [Member] | Sales Comparison Approach [Member] | Minimum [Member] | Foreclosed Assets [Member] | Commercial Real Estate [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	(65.00%)	(84.00%)
Level 3 [Member] | Sales Comparison Approach [Member] | Maximum [Member] | Impaired Originated & PNCI Loans [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	60.00%	23.00%
Level 3 [Member] | Sales Comparison Approach [Member] | Maximum [Member] | Foreclosed Assets [Member] | Land & Construction [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average		283.00%
Level 3 [Member] | Sales Comparison Approach [Member] | Maximum [Member] | Foreclosed Assets [Member] | Residential Real Estate [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	52.00%	39.00%
Level 3 [Member] | Sales Comparison Approach [Member] | Maximum [Member] | Foreclosed Assets [Member] | Commercial Real Estate [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	20.00%	19.00%
Level 3 [Member] | Sales Comparison Approach [Member] | Weighted Average [Member] | Impaired Originated & PNCI Loans [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	(28.00%)	(19.76%)
Level 3 [Member] | Sales Comparison Approach [Member] | Weighted Average [Member] | Foreclosed Assets [Member] | Land & Construction [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average		167.00%
Level 3 [Member] | Sales Comparison Approach [Member] | Weighted Average [Member] | Foreclosed Assets [Member] | Residential Real Estate [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	0.90%	(3.13%)
Level 3 [Member] | Sales Comparison Approach [Member] | Weighted Average [Member] | Foreclosed Assets [Member] | Commercial Real Estate [Member] | Adjustment for Differences between Comparable Sales [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Weighted average	(45.00%)	(84.00%)