Fair Value Measurement - Quantitative Information about Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Nonrecurring Basis (Detail) $ in Thousands	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Impaired Originated & PNCI Loans [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,647	$ 2,767
Foreclosed Assets [Member] | Land & Construction [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	492	1,341
Foreclosed Assets [Member] | Residential Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 92	622
Foreclosed Assets [Member] | Commercial Real Estate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value		$ 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	(0.7000)	(0.7400)
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	(0.4700)	(0.53)
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	(0.65)	(0.4700)
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		(0.84)
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	0.6000	0.2300
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	0.3900	2.83
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	0.20	0.3900
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		0.19
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	(0.0741)	(0.1976)
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	0.0364	1.67
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	(0.45)	(0.0313)
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		(0.84)