Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	$ (2,150)	$ (1,859)
Securities available for sale, Less Than Twelve Months, Fair Value	354,205	239,768
Securities available for sale, Over Twelve Months, Gross Unrealized Losses	(180)	(187)
Securities available for sale, Over Twelve Months, Fair Value	3,773	5,013
Securities available for sale, Gross Unrealized Losses, Total	(2,330)	(2,046)
Securities available for sale, Fair Value, Total	357,978	244,781
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses	(678)	(266)
Securities held to maturity, Less Than Twelve Months, Fair Value	34,850	24,504
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses
Securities held to maturity, Over Twelve Months, Fair Value
Securities held to maturity, Gross Unrealized Losses, Total	(678)	(266)
Securities held to maturity, Fair Value, Total	34,850	24,504
U.S. Government-sponsored enterprise obligations [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	(298)	(386)
Securities available for sale, Less Than Twelve Months, Fair Value	165,763	59,741
Securities available for sale, Gross Unrealized Losses, Total	(298)	(386)
Securities available for sale, Fair Value, Total	165,763	59,741
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses
Securities held to maturity, Less Than Twelve Months, Fair Value
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses
Securities held to maturity, Over Twelve Months, Fair Value
Securities held to maturity, Gross Unrealized Losses, Total
Securities held to maturity, Fair Value, Total
Obligations of state and political subdivisions [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	(1)
Securities available for sale, Less Than Twelve Months, Fair Value	336
Securities available for sale, Over Twelve Months, Gross Unrealized Losses	(178)	(178)
Securities available for sale, Over Twelve Months, Fair Value	1,094	1,094
Securities available for sale, Gross Unrealized Losses, Total	(179)	(178)
Securities available for sale, Fair Value, Total	1,430	1,094
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses	(232)	(113)
Securities held to maturity, Less Than Twelve Months, Fair Value	10,478	8,242
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses
Securities held to maturity, Over Twelve Months, Fair Value
Securities held to maturity, Gross Unrealized Losses, Total	(232)	(113)
Securities held to maturity, Fair Value, Total	10,478	8,242
Mortgage backed securities [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Less Than Twelve Months, Gross Unrealized Losses	(1,851)	(1,473)
Securities available for sale, Less Than Twelve Months, Fair Value	188,106	180,027
Securities available for sale, Over Twelve Months, Gross Unrealized Losses	(2)	(9)
Securities available for sale, Over Twelve Months, Fair Value	2,679	3,919
Securities available for sale, Gross Unrealized Losses, Total	(1,853)	(1,482)
Securities available for sale, Fair Value, Total	190,785	183,946
Securities held to maturity, Less Than Twelve Months, Gross Unrealized Losses	(446)	(153)
Securities held to maturity, Less Than Twelve Months, Fair Value	24,372	16,262
Securities held to maturity, Over Twelve Months, Gross Unrealized Losses
Securities held to maturity, Over Twelve Months, Fair Value
Securities held to maturity, Gross Unrealized Losses, Total	(446)	(153)
Securities held to maturity, Fair Value, Total	$ 24,372	$ 16,262

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.

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Amount of the excess of amortized cost basis over fair value of securities in a loss position and categorized neither as held-to-maturity nor trading securities.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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