Allowance for Loan Losses - Loans by Credit Ratings Issued by National Organizations (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	$ 302,193
Commercial and Industrial [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	180,006
Commercial and Industrial [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	112,507
Commercial and Industrial [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	9,680
Municipal [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	80,543
Municipal [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	59,628
Municipal [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	7,400
Municipal [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	9,035
Municipal [Member] | Ba2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	4,480
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	293,040
Commercial Real Estate [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	7,781
Commercial Real Estate [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	131,230
Commercial Real Estate [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	$ 154,029