Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Specific loss allocations are calculated for commercial loans in excess					$ 500,000,000
Number of days for collecting principal or interest on consumer loans					90 days
Minimum collection period for home equity loan or high loan to value loan					180 days
Retail credits amount deemed unrecoverable charged-off period					60 days
Retail credits charged off period after discovery of the fraud					90 days
Reversal of provision for loan losses related to loans acquired		$ 738,000		$ 371,000
provision for loan losses related to loans acquired	$ 801		$ 2,080
Reserve for lending-related commitments	$ 927,000		$ 927,000		$ 679,000
Closed-End Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days delinquent from contractual due date					12 days
Open-End Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days delinquent from contractual due date					180 days