Fair Value Measurement - Estimated Fair Values of Financial Instruments that are Reported at Amortized Cost in Consolidated Balance Sheets (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Financial assets:
Cash and due from banks	$ 82,610,000	$ 94,305,000
Cash at Federal Reserve and other banks	232,478,000	209,156,000
Securities held to maturity	641,149,000	726,530,000
Financial liabilities:
Other borrowings	19,235,000	12,328,000
Junior subordinated debt	56,617,000	56,470,000
Level 3 [Member]
Financial assets:
Securities held to maturity	0	0
Contract Amount [Member] | Level 3 [Member]
Off-balance sheet:
Commitments	805,284,000	705,316,000
Standby letters of credit	10,349,000	8,330,000
Overdraft privilege commitments	98,400,000	94,473,000
Fair Value [Member] | Level 3 [Member]
Financial assets:
Loans, net	2,761,975,000	2,555,297,000
Financial liabilities:
Junior subordinated debt	51,806,000	44,527,000
Off-balance sheet:
Commitments	8,053,000	7,053,000
Standby letters of credit	103,000	83,000
Overdraft privilege commitments	984,000	945,000
Fair Value [Member] | Level 1 [Member]
Financial assets:
Cash and due from banks	82,610,000	94,305,000
Cash at Federal Reserve and other banks	232,478,000	209,156,000
Fair Value [Member] | Level 2 [Member]
Financial assets:
Securities held to maturity	663,781,000	732,208,000
Loans held for sale	7,777,000	1,873,000
Financial liabilities:
Deposits	3,835,424,000	3,630,129,000
Other borrowings	19,235,000	12,328,000
Carrying Amount [Member] | Level 3 [Member]
Financial assets:
Loans, net	2,678,742,000	2,486,926,000
Financial liabilities:
Junior subordinated debt	56,617,000	56,470,000
Carrying Amount [Member] | Level 1 [Member]
Financial assets:
Cash and due from banks	82,610,000	94,305,000
Cash at Federal Reserve and other banks	232,478,000	209,156,000
Carrying Amount [Member] | Level 2 [Member]
Financial assets:
Securities held to maturity	641,149,000	726,530,000
Restricted equity securities	16,956,000	16,956,000
Loans held for sale	7,777,000	1,873,000
Financial liabilities:
Deposits	3,836,012,000	3,631,266,000
Other borrowings	$ 19,235,000	$ 12,328,000