Investment Securities - Amortized Cost and Estimated Fair Value of Held-to-Maturity Securities and Available-for-Sale Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Held-to-Maturity, Fair Value	$ 70,079	$ 71,327	[1]
Total Amortized Cost	393,636	416,898
Total Gross Unrealized Gains	3,950	13,097
Total Gross Unrealized Losses	(4,227)	(261)
Total Fair Value	393,359	429,734	[1]
Corporate bonds [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost Within 1 year	12,228	3,026
Held-to-Maturity, Gross Unrealized Gains Within 1 year	86	7
Held-to-Maturity, Gross Unrealized Losses Within 1 year
Held-to-Maturity, Fair Value Within 1 year	12,314	3,033
Held-to-Maturity After 1 year to 5 years, Amortized Cost	56,986	66,819
Held-to-Maturity After 1 year to 5 years, Gross Unrealized Gains	1,040	1,526
Held-to-Maturity After 1 year to 5 years, Gross Unrealized Losses	(261)	(51)
Held-to-Maturity After 1 year to 5 years, Fair Value	57,765	68,294
Held-to-Maturity, Amortized Cost	69,214	69,845
Held-to-Maturity, Gross Unrealized Gains	1,126	1,533
Held-to-Maturity, Gross Unrealized Losses	(261)	(51)
Held-to-Maturity, Fair Value	70,079	71,327
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	10,819	4,993
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	35	21
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years	(131)
Available-for-Sale Securities, Fair Value After 1 year to 5 years	10,723	5,014
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	22,833
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years	8
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(1,156)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	21,685
Total Amortized Cost	33,652	4,993
Total Gross Unrealized Gains	43	21
Total Gross Unrealized Losses	(1,287)
Available-for-Sale Securities, Debt Total Fair Value	32,408	5,014
U.S. treasuries [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	4,965	4,960
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(192)	(22)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	4,773	4,938
Total Amortized Cost	4,965	4,960
Total Gross Unrealized Gains
Total Gross Unrealized Losses	(192)	(22)
Available-for-Sale Securities, Debt Total Fair Value	4,773	4,938
U.S. government corporations and agencies [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities, Amortized Cost Within 1 year	5,999	1,517
Available-for-Sale Securities, Gross Unrealized Gains Within 1 year	36	9
Available-for-Sale Securities, Gross Unrealized Losses Within 1 year
Available-for-Sale Securities, Fair Value Within 1 year	6,035	1,526
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	148,123	148,120
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	169	1,509
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years	(1,157)	(70)
Available-for-Sale Securities, Fair Value After 1 year to 5 years	147,135	149,559
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	10,850	20,953
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years		109
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(471)	(5)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	10,379	21,057
Total Amortized Cost	164,972	170,590
Total Gross Unrealized Gains	205	1,627
Total Gross Unrealized Losses	(1,628)	(75)
Available-for-Sale Securities, Debt Total Fair Value	163,549	172,142
State and political subdivisions [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities, Amortized Cost Within 1 year	3,825	4,607
Available-for-Sale Securities, Gross Unrealized Gains Within 1 year	14	75
Available-for-Sale Securities, Gross Unrealized Losses Within 1 year
Available-for-Sale Securities, Fair Value Within 1 year	3,839	4,682
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	4,603	4,130
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	29	88
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years	(35)	(19)
Available-for-Sale Securities, Fair Value After 1 year to 5 years	4,597	4,199
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	41,545	36,499
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years	720	1,245
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(530)	(7)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	41,735	37,737
Available-for-Sale Securities, Amortized Cost Over 10 years	65,746	70,495
Available-for-Sale Securities, Gross Unrealized Gains Over 10 years	1,788	5,055
Available-for-Sale Securities, Gross Unrealized Losses Over 10 years	(311)
Available-for-Sale Securities, Fair Value Over 10 years	67,223	75,550
Total Amortized Cost	115,719	115,731
Total Gross Unrealized Gains	2,551	6,463
Total Gross Unrealized Losses	(876)	(26)
Available-for-Sale Securities, Debt Total Fair Value	117,394	122,168
Residential mortgage-backed securities [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	18,365	20,140
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years	307	777
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(55)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	18,617	20,917
Available-for-Sale Securities, Amortized Cost Over 10 years	35,075	66,962
Available-for-Sale Securities, Gross Unrealized Gains Over 10 years	375	2,861
Available-for-Sale Securities, Gross Unrealized Losses Over 10 years
Available-for-Sale Securities, Fair Value Over 10 years	35,450	69,823
Total Amortized Cost	53,440	87,102
Total Gross Unrealized Gains	682	3,638
Total Gross Unrealized Losses	(55)
Available-for-Sale Securities, Debt Total Fair Value	54,067	90,740
Collateralized mortgage obligations [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	109	41
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	1
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years
Available-for-Sale Securities, Fair Value After 1 year to 5 years	110	41
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years		626
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years		7
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years
Available-for-Sale Securities, Fair Value After 5 years to 10 years		633
Available-for-Sale Securities, Amortized Cost Over 10 years	9,462	25,698
Available-for-Sale Securities, Gross Unrealized Gains Over 10 years	111	645
Available-for-Sale Securities, Gross Unrealized Losses Over 10 years	(163)	(5)
Available-for-Sale Securities, Fair Value Over 10 years	9,410	26,338
Total Amortized Cost	9,571	26,365
Total Gross Unrealized Gains	112	652
Total Gross Unrealized Losses	(163)	(5)
Available-for-Sale Securities, Debt Total Fair Value	9,520	27,012
Money market mutual funds [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities, Amortized Cost Within 1 year	9,639	4,878
Available-for-Sale Securities, Gross Unrealized Gains Within 1 year
Available-for-Sale Securities, Gross Unrealized Losses Within 1 year
Available-for-Sale Securities, Fair Value Within 1 year	9,639	4,878
Total Amortized Cost	9,639	4,878
Total Gross Unrealized Gains
Total Gross Unrealized Losses
Available-for-Sale Securities, Debt Total Fair Value	9,639	4,878
Equity securities [Member]
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Available-for-Sale Securities Equity, No stated maturity Amortized Cost	1,678	2,279
Available-for-Sale Securities Equity, No stated maturity Gross Unrealized Gains	357	696
Available For Sale Securities Equity No stated maturity Gross Unrealized Losses	(26)	(133)
Available-for-Sale Securities Equity, No stated maturity Fair Value	2,009	2,842
Available-for-Sale Securities Equity, Total Amortized Cost	1,678	2,279
Available-for-Sale Securities Equity, Total Gross Unrealized Gains	357	696
Available-for-Sale Securities Equity, Total Gross Unrealized Losses	(26)	(133)
Available-for-Sale Securities, Equity Total Fair Value	$ 2,009	$ 2,842

[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.