Fair Value Disclosures - Assets and liabilities measured at fair value on a recurring basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Available-for-sale securities:
Total available-for-sale securities	$ 416,035	$ 429,734	[1]
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	416,807	431,281
Total liabilities	529	2,866
Available-for-sale securities:
Total available-for-sale securities	416,035	429,734
Fair Value, Measurements, Recurring [Member] | U.S. treasuries [Member]
Available-for-sale securities:
Total available-for-sale securities	4,763	4,938
Fair Value, Measurements, Recurring [Member] | U.S. government corporations and agencies [Member]
Available-for-sale securities:
Total available-for-sale securities	173,421	172,142
Fair Value, Measurements, Recurring [Member] | State and political subdivisions [Member]
Available-for-sale securities:
Total available-for-sale securities	119,934	122,168
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Available-for-sale securities:
Total available-for-sale securities	67,923	90,740
Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations [Member]
Available-for-sale securities:
Total available-for-sale securities	10,474	27,012
Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member]
Available-for-sale securities:
Total available-for-sale securities	31,008	5,014
Fair Value, Measurements, Recurring [Member] | Money market mutual funds [Member]
Available-for-sale securities:
Total available-for-sale securities	5,500	4,878
Fair Value, Measurements, Recurring [Member] | Equity securities [Member]
Available-for-sale securities:
Total available-for-sale securities	3,012	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		1,547
Liabilities:
Liabilities	64
Fair Value, Measurements, Recurring [Member] | Interest rate swap [Member]
Liabilities:
Liabilities		1,909
Fair Value, Measurements, Recurring [Member] | Forward loan sale commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	772
Liabilities:
Liabilities		54
Fair Value, Measurements, Recurring [Member] | Contingent consideration liability [Member]
Liabilities:
Liabilities	465	903
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	13,275	12,658
Available-for-sale securities:
Total available-for-sale securities	13,275	12,658
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. treasuries [Member]
Available-for-sale securities:
Total available-for-sale securities	4,763	4,938
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money market mutual funds [Member]
Available-for-sale securities:
Total available-for-sale securities	5,500	4,878
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Equity securities [Member]
Available-for-sale securities:
Total available-for-sale securities	3,012	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	403,532	418,623
Total liabilities	64	1,963
Available-for-sale securities:
Total available-for-sale securities	402,760	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	173,421	172,142
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | State and political subdivisions [Member]
Available-for-sale securities:
Total available-for-sale securities	119,934	122,168
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Available-for-sale securities:
Total available-for-sale securities	67,923	90,740
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Collateralized mortgage obligations [Member]
Available-for-sale securities:
Total available-for-sale securities	10,474	27,012
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate bonds [Member]
Available-for-sale securities:
Total available-for-sale securities	31,008	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		1,547
Liabilities:
Liabilities	64
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]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	772
Liabilities:
Liabilities		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	465	903
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Contingent consideration liability [Member]
Liabilities:
Liabilities	$ 465	$ 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.