Fair Value Disclosures - Assets Measured at Fair Value on Non-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]
Total assets	$ 1,958	$ 2,138
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held for investment	28,351	43,680
Impaired leases held for investment	1,250
Other real estate owned	1,843	4,969
Total assets	30,194	48,649
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held for investment	0	0
Impaired leases held for investment	0
Other real estate owned	0	0
Total assets	0	0
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held for investment	0	0
Impaired leases held for investment	0
Other real estate owned	0	0
Total assets	0	0
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans held for investment	28,351	43,680
Impaired leases held for investment	1,250
Other real estate owned	1,843	4,969
Total assets	$ 30,194	$ 48,649