Fair Value Disclosures - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale securities:
Total available-for-sale securities	$ 393,359	$ 429,734	[1]
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	394,208	431,281
Total liabilities	844	2,866
Available-for-sale securities:
Total available-for-sale securities	393,359	429,734
Fair Value, Measurements, Recurring [Member] | U.S. treasuries [Member]
Available-for-sale securities:
Total available-for-sale securities	4,773	4,938
Fair Value, Measurements, Recurring [Member] | U.S. government corporations and agencies [Member]
Available-for-sale securities:
Total available-for-sale securities	163,549	172,142
Fair Value, Measurements, Recurring [Member] | State and political subdivisions [Member]
Available-for-sale securities:
Total available-for-sale securities	117,394	122,168
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Available-for-sale securities:
Total available-for-sale securities	54,067	90,740
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations [Member]
Available-for-sale securities:
Total available-for-sale securities	9,520	27,012
Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member]
Available-for-sale securities:
Total available-for-sale securities	32,408	5,014
Fair Value, Measurements, Recurring [Member] | Money market mutual funds [Member]
Available-for-sale securities:
Total available-for-sale securities	9,639	4,878
Fair Value, Measurements, Recurring [Member] | Equity securities [Member]
Available-for-sale securities:
Total available-for-sale securities	2,009	2,842
Fair Value, Measurements, Recurring [Member] | Interest Rate Locks with Customers [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	849	1,547
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Liabilities:
Liabilities		1,909
Fair Value, Measurements, Recurring [Member] | Forward Loan Sale Commitments [Member]
Liabilities:
Liabilities	361	54
Fair Value, Measurements, Recurring [Member] | Contingent consideration liability [Member]
Liabilities:
Liabilities	483	903
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	16,421	12,658
Available-for-sale securities:
Total available-for-sale securities	16,421	12,658
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. treasuries [Member]
Available-for-sale securities:
Total available-for-sale securities	4,773	4,938
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money market mutual funds [Member]
Available-for-sale securities:
Total available-for-sale securities	9,639	4,878
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Equity securities [Member]
Available-for-sale securities:
Total available-for-sale securities	2,009	2,842
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	377,787	418,623
Total liabilities	361	1,963
Available-for-sale securities:
Total available-for-sale securities	376,938	417,076
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. government corporations and agencies [Member]
Available-for-sale securities:
Total available-for-sale securities	163,549	172,142
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | State and political subdivisions [Member]
Available-for-sale securities:
Total available-for-sale securities	117,394	122,168
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Available-for-sale securities:
Total available-for-sale securities	54,067	90,740
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations [Member]
Available-for-sale securities:
Total available-for-sale securities	9,520	27,012
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member]
Available-for-sale securities:
Total available-for-sale securities	32,408	5,014
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Locks with Customers [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	849	1,547
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Liabilities:
Liabilities		1,909
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Forward Loan Sale Commitments [Member]
Liabilities:
Liabilities	361	54
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Total liabilities	483	903
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Contingent consideration liability [Member]
Liabilities:
Liabilities	$ 483	$ 903

[1] Note: The consolidated balance sheet at December 31, 2012 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U. S. generally accepted accounting principles for complete financial statements. See accompanying notes to the unaudited consolidated financial statements.