Fair Values of Financial Instruments - Carrying Amounts and Fair Values of Company's Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financial assets:
Securities held-to-maturity	$ 267,613	$ 275,507
Loans	1,143,232	1,092,591
Financial liabilities:
Time deposits	466,492	419,983
Other borrowed funds	177,144	195,144
Subordinated debentures	36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	267,613	275,507
Loans	1,143,232	1,092,591
Financial liabilities:
Time deposits	466,492	419,983
Other borrowed funds	177,144	195,144
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	268,802	281,924
Loans	1,135,920	1,124,716
Financial liabilities:
Time deposits	469,342	424,253
Other borrowed funds	181,428	205,481
Subordinated debentures	41,129	43,423
Fair Value Measurements, Level 1 Inputs [Member]
Financial assets:
Securities held-to-maturity
Loans
Financial liabilities:
Time deposits
Other borrowed funds
Subordinated debentures
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	268,802	281,924
Financial liabilities:
Time deposits	469,342	424,253
Other borrowed funds	181,428	205,481
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	1,135,920	1,124,716
Financial liabilities:
Subordinated debentures	$ 41,129	$ 43,423