Loans and Allowance for Loan Losses - Schedule of Activity of Purchased Impaired Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	$ 39,477	$ 43,526
Allowance for loan losses	(326)	(166)
Carrying amount, net of allowance	39,151	43,360
Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	4,359	5,152
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	20,561	21,826
Residential [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	4,242	11,303
Consumer Loan [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	$ 10,315	$ 5,245