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Investment Securities - Additional Information (Detail) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2016 USD ($) position | Mar. 31, 2015 USD ($) | Dec. 31, 2015 position
Investment [Line Items] |  |  |
Percentage of debt securities considered to be of investment-grade | 98.00% |  |
Percentage of tax-exempt municipal securities classified as pre-refunded | 6.00% |  |
General obligation bonds as percentage of tax-exempt municipal securities that were not pre-refunded | 46.00% |  |
Special revenue bonds as percentage of tax exempt municipal securities that were not pre-refunded | 54.00% |  |
Percentage of tax exempt securities | 5.40% |  |
Maximum percentage of any general obligation bonds in one state | 9.00% |  |
Percentage of corporate securities associated with financial services industry | 24.00% |  | 25.00%
Securities in unrealized loss positions, number of positions | 360 |  |
Number of positions |  |  | 2,000
Other-than-temporary impairments | $ | $ 0 | $ 0 |
Residential Mortgage-Backed Securities |  |  |
Investment [Line Items] |  |  |
Mortgage-backed Securities as Percentage of Agency Mortgage-backed Securities | 98.00% |  | 98.00%

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