Acquired SJB Assets and FDIC Loss Sharing Asset - Summary of Gross PCI Loans by Internal Risk Ratings by Loans (Detail) - PCI Loans [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	$ 17,260	$ 30,908
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	15,775	26,439
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	1,251	1,088
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	234	3,381
Doubtful & Loss [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	$ 0	$ 0