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	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available for Sale and Held To Maturity Securities [Line Items]
Held-to-Maturity, Fair Value	$ 70,029	$ 71,327	[1]
Total Amortized Cost	415,326	416,898
Total Gross Unrealized Gains	5,609	13,097
Total Gross Unrealized Losses	(4,900)	(261)
Total Fair Value	416,035	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	12,313	3,026
Held-to-Maturity, Gross Unrealized Gains Within 1 year	118	7
Held-to-Maturity, Gross Unrealized Losses Within 1 year
Held-to-Maturity, Fair Value Within 1 year	12,431	3,033
Held-to-Maturity After 1 year to 5 years, Amortized Cost	57,112	66,819
Held-to-Maturity After 1 year to 5 years, Gross Unrealized Gains	969	1,526
Held-to-Maturity After 1 year to 5 years, Gross Unrealized Losses	(483)	(51)
Held-to-Maturity After 1 year to 5 years, Fair Value	57,598	68,294
Held-to-Maturity, Amortized Cost	69,425	69,845
Held-to-Maturity, Gross Unrealized Gains	1,087	1,533
Held-to-Maturity, Gross Unrealized Losses	(483)	(51)
Held-to-Maturity, Fair Value	70,029	71,327
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	10,081	4,993
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	23	21
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years	(207)
Available-for-Sale Securities, Fair Value After 1 year to 5 years	9,897	5,014
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	22,179
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(1,068)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	21,111
Total Amortized Cost	32,260	4,993
Total Gross Unrealized Gains	23	21
Total Gross Unrealized Losses	(1,275)
Available-for-Sale Securities, Debt Total Fair Value	31,008	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	4,963	4,960
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(200)	(22)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	4,763	4,938
Total Amortized Cost	4,963	4,960
Total Gross Unrealized Gains
Total Gross Unrealized Losses	(200)	(22)
Available-for-Sale Securities, Debt Total Fair Value	4,763	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	5,998	1,517
Available-for-Sale Securities, Gross Unrealized Gains Within 1 year	51	9
Available-for-Sale Securities, Gross Unrealized Losses Within 1 year
Available-for-Sale Securities, Fair Value Within 1 year	6,049	1,526
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	153,254	148,120
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	490	1,509
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years	(1,649)	(70)
Available-for-Sale Securities, Fair Value After 1 year to 5 years	152,095	149,559
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	15,885	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	(608)	(5)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	15,277	21,057
Total Amortized Cost	175,137	170,590
Total Gross Unrealized Gains	541	1,627
Total Gross Unrealized Losses	(2,257)	(75)
Available-for-Sale Securities, Debt Total Fair Value	173,421	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	5,383	4,607
Available-for-Sale Securities, Gross Unrealized Gains Within 1 year	34	75
Available-for-Sale Securities, Gross Unrealized Losses Within 1 year
Available-for-Sale Securities, Fair Value Within 1 year	5,417	4,682
Available-for-Sale Securities, Amortized Cost After 1 year to 5 years	3,925	4,130
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	40	88
Available-for-Sale Securities, Gross Unrealized Losses After 1 year to 5 years	(36)	(19)
Available-for-Sale Securities, Fair Value After 1 year to 5 years	3,929	4,199
Available-for-Sale Securities, Amortized Cost After 5 years to 10 years	41,966	36,499
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years	844	1,245
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(562)	(7)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	42,248	37,737
Available-for-Sale Securities, Amortized Cost Over 10 years	66,555	70,495
Available-for-Sale Securities, Gross Unrealized Gains Over 10 years	2,098	5,055
Available-for-Sale Securities, Gross Unrealized Losses Over 10 years	(313)
Available-for-Sale Securities, Fair Value Over 10 years	68,340	75,550
Total Amortized Cost	117,829	115,731
Total Gross Unrealized Gains	3,016	6,463
Total Gross Unrealized Losses	(911)	(26)
Available-for-Sale Securities, Debt Total Fair Value	119,934	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	22,236	20,140
Available-for-Sale Securities, Gross Unrealized Gains After 5 years to 10 years	468	777
Available-for-Sale Securities, Gross Unrealized Losses After 5 years to 10 years	(73)
Available-for-Sale Securities, Fair Value After 5 years to 10 years	22,631	20,917
Available-for-Sale Securities, Amortized Cost Over 10 years	44,750	66,962
Available-for-Sale Securities, Gross Unrealized Gains Over 10 years	617	2,861
Available-for-Sale Securities, Gross Unrealized Losses Over 10 years	(75)
Available-for-Sale Securities, Fair Value Over 10 years	45,292	69,823
Total Amortized Cost	66,986	87,102
Total Gross Unrealized Gains	1,085	3,638
Total Gross Unrealized Losses	(148)
Available-for-Sale Securities, Debt Total Fair Value	67,923	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	155	41
Available-for-Sale Securities, Gross Unrealized Gains After 1 year to 5 years	3
Available-for-Sale Securities, Fair Value After 1 year to 5 years	158	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, Fair Value After 5 years to 10 years		633
Available-for-Sale Securities, Amortized Cost Over 10 years	10,335	25,698
Available-for-Sale Securities, Gross Unrealized Gains Over 10 years	74	645
Available-for-Sale Securities, Gross Unrealized Losses Over 10 years	(93)	(5)
Available-for-Sale Securities, Fair Value Over 10 years	10,316	26,338
Total Amortized Cost	10,490	26,365
Total Gross Unrealized Gains	77	652
Total Gross Unrealized Losses	(93)	(5)
Available-for-Sale Securities, Debt Total Fair Value	10,474	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	5,500	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	5,500	4,878
Total Amortized Cost	5,500	4,878
Total Gross Unrealized Gains
Total Gross Unrealized Losses
Available-for-Sale Securities, Debt Total Fair Value	5,500	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	2,161	2,279
Available-for-Sale Securities Equity, No stated maturity Gross Unrealized Gains	867	696
Available For Sale Securities Equity No stated maturity Gross Unrealized Losses	(16)	(133)
Available-for-Sale Securities Equity, No stated maturity Fair Value	3,012	2,842
Available-for-Sale Securities Equity, Total Amortized Cost	2,161	2,279
Available-for-Sale Securities Equity, Total Gross Unrealized Gains	867	696
Available-for-Sale Securities Equity, Total Gross Unrealized Losses	(16)	(133)
Available-for-Sale Securities, Equity Total Fair Value	$ 3,012	$ 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.