Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Percentage of debt securities considered to be of investment-grade	94.00%		94.00%
Percentage of tax-exempt municipal securities classified as pre-refunded	10.00%		10.00%
General obligation bonds as percentage of tax-exempt municipal securities that were not pre-refunded	41.00%
Special revenue bonds as percentage of tax exempt municipal securities that were not pre-refunded	59.00%
Maximum percentage of any general obligation bonds in one state	10.00%
Percentage of tax-exempt securities insured by bond insurers	20.00%		20.00%
Residential and commercial mortgage-backed securities, percentage of collateral consisting of prime loans	99.00%		99.00%
Percentage of corporate securities associated with financial services industry	24.00%		24.00%		23.00%
Other-than-temporary impairments	$ 0	$ 0	$ 0	$ 0

General Obligation Bonds As Percentage Of Tax Exempt Municipal Securities That Were Not Pre Refunded

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Maximum percentage of any general obligation bonds in one state

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Percentage of collateral related to prime loans

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Percentage of corporate securities associated with financial services industry.

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Percentage of tax exempt securities

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Percentage Of tax-exempt to municipal securities classified as pre-refunded

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- Definition

Percentage of Total Debt Securities

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Special Revenue Bonds As Percentage Of Tax Exempt Municipal Securities That Were Not Pre Refunded

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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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