Fair Value Measurement - Estimated Fair Values of Financial Instruments that are Reported at Amortized Cost in Consolidated Balance Sheets (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Cash and due from banks	$ 105,968,000	$ 92,197,000
Cash at Federal Reserve and other banks	99,460,000	213,415,000
Securities held to maturity	514,844,000	602,536,000
Financial liabilities:
Other borrowings	122,166,000	17,493,000
Junior subordinated debt	56,858,000	56,667,000
Level 3 [Member]
Financial assets:
Securities held to maturity	0	0
Level 3 [Member] | Overdraft Privilege Commitments [Member]
Off-balance sheet:
Contract amount, Off-balance sheet	98,260,000	98,583,000
Level 3 [Member] | Standby Letters of Credit [Member]
Off-balance sheet:
Contract amount, Off-balance sheet	13,075,000	12,763,000
Level 3 [Member] | Commitments [Member]
Off-balance sheet:
Contract amount, Off-balance sheet	933,542,000	767,274,000
Fair Value [Member] | Level 3 [Member]
Financial assets:
Loans, net	2,992,225,000	2,763,473,000
Financial liabilities:
Junior subordinated debt	58,466,000	49,033,000
Fair Value [Member] | Level 3 [Member] | Overdraft Privilege Commitments [Member]
Off-balance sheet:
Fair value, Off-balance sheet	983,000	986,000
Fair Value [Member] | Level 3 [Member] | Standby Letters of Credit [Member]
Off-balance sheet:
Fair value, Off-balance sheet	131,000	128,000
Fair Value [Member] | Level 3 [Member] | Commitments [Member]
Off-balance sheet:
Fair value, Off-balance sheet	9,335,000	7,673,000
Fair Value [Member] | Level 1 [Member]
Financial assets:
Cash and due from banks	105,968,000	92,197,000
Cash at Federal Reserve and other banks	99,460,000	213,415,000
Fair Value [Member] | Level 2 [Member]
Financial assets:
Securities held to maturity	518,165,000	603,203,000
Loans held for sale	4,616,000	2,998,000
Financial liabilities:
Deposits	4,006,620,000	3,893,941,000
Other borrowings	122,166,000	17,493,000
Carrying Amount [Member] | Level 3 [Member]
Financial assets:
Loans, net	2,984,842,000	2,727,090,000
Financial liabilities:
Junior subordinated debt	56,858,000	56,667,000
Carrying Amount [Member] | Level 1 [Member]
Financial assets:
Cash and due from banks	105,968,000	92,197,000
Cash at Federal Reserve and other banks	99,460,000	213,415,000
Carrying Amount [Member] | Level 2 [Member]
Financial assets:
Securities held to maturity	514,844,000	602,536,000
Restricted equity securities	16,956,000	16,596,000
Loans held for sale	4,616,000	2,998,000
Financial liabilities:
Deposits	4,009,131,000	3,895,560,000
Other borrowings	$ 122,166,000	$ 17,493,000