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Loans - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($) Investment	Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($) Investment	Dec. 31, 2017 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Outstanding loans to Executive officers, directors, principal shareholders, related interest and parties	$ 55,300,000		$ 55,300,000
Loans held for sale	0		0		$ 35,258,000
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates		$ 16,272,000	$ 5,286,000	$ 33,315,000
Number of loans classified as a non-accrual TDRs | Investment		24	19	58
Commercial and Industrial
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates	$ 1,212,000	$ 7,620,000	$ 4,386,000	$ 23,673,000
Number of loans classified as a non-accrual TDRs | Investment	6	22	18	52
Commercial and Industrial | Payment Default
Financing Receivable, Recorded Investment [Line Items]
Number of loans classified as a non-accrual TDRs | Investment			5
Loan classified as non accrual TDRs	$ 1,100,000		$ 1,100,000
Principal shareholders
Financing Receivable, Recorded Investment [Line Items]
Outstanding loans to Executive officers, directors, principal shareholders, related interest and parties	$ 0		0
Financing Receivable Troubled Debt Restructurings Rate Reductions
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates			38,700,000
Financing Receivable Troubled Debt Restructurings Forbearance of Arrears
Financing Receivable, Recorded Investment [Line Items]
Delinquent loans selectively extended to certain borrowers, rate reductions, forbearance of arrears, and extension of maturity dates			$ 1,400,000

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