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, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	$ 37,306	$ 73,093
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	32,309	59,409
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	147	1,162
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	4,850	12,522
Doubtful & Loss [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross PCI loans	$ 0	$ 0