Fair Values of Assets and Liabilities - Carrying Amounts and Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Carrying Amount [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	$ 98,319	$ 91,033
Interest-bearing deposits	29,709	24,644
Securities available for sale, at fair value	778,644	778,612
Held-to-maturity Securities	436,081	372,513
Other investments	32,302
Loans Held for sale, Amount	17,100	7,052
Loans and Leases Receivable, Net of Deferred Income	5,375,847	4,879,347
Deposits	5,555,057	4,947,408
Short-term borrowings	805,825	867,943
Subordinated debt	72,167	72,167
Long-term debt	8,311	8,749
Capital Lease Obligations	7,677
Estimate of Fair Value Measurement [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	98,319	91,033
Interest-bearing deposits	29,709	24,644
Securities available for sale, at fair value	778,644	778,612
Held-to-maturity Securities	434,882	368,618
Other investments	32,302	36,498
Loans Held for sale, Amount	17,100	7,052
Loans and Leases Receivable, Net of Deferred Income	5,403,523	4,878,254
Deposits	5,556,870	4,949,714
Short-term borrowings	805,721	867,667
Subordinated debt	66,779	65,656
Long-term debt	8,805	9,169
Capital Lease Obligations	7,677
Estimate of Fair Value Measurement [Member] | Level 1 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	98,319	91,033
Interest-bearing deposits	29,709	24,644
Securities available for sale, at fair value	0	0
Held-to-maturity Securities	0	0
Other investments	0	0
Loans Held for sale, Amount	0	0
Loans and Leases Receivable, Net of Deferred Income	0	0
Deposits	0	0
Short-term borrowings	0	0
Subordinated debt	0	0
Long-term debt	0	0
Capital Lease Obligations	0
Estimate of Fair Value Measurement [Member] | Level 2 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	0	0
Interest-bearing deposits	0	0
Securities available for sale, at fair value	742,345	743,650
Held-to-maturity Securities	434,882	368,618
Other investments	32,302	36,498
Loans Held for sale, Amount	17,100	7,052
Loans and Leases Receivable, Net of Deferred Income	19,878	18,679
Deposits	5,556,870	4,949,714
Short-term borrowings	805,721	867,667
Subordinated debt	0	0
Long-term debt	8,805	9,169
Capital Lease Obligations	7,677
Estimate of Fair Value Measurement [Member] | Level 3 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	0	0
Interest-bearing deposits	0	0
Securities available for sale, at fair value	36,299	34,962
Held-to-maturity Securities	0	0
Other investments	0	0
Loans Held for sale, Amount	0	0
Loans and Leases Receivable, Net of Deferred Income	5,383,645	4,859,575
Deposits	0	0
Short-term borrowings	0	0
Subordinated debt	66,779	65,656
Long-term debt	0	$ 0
Capital Lease Obligations	$ 0