| **Impairment of Investment Securities - Additional Information (Detail) (USD $)**                         | 3 Months Ended  | 9 Months Ended |                 |
|-----------------------------------------------------------------------------------------------------------|-----------------|----------------|-----------------|
||
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Other than temporary impairment charges recognized                                                        |            |           | $ 0             |  | $ 0 |  |   |
| Non-credit related gains on securities not expected to be sold (recognized in other comprehensive income) | (236,000)       | (1,374,000)    | (4,941,000)     |  | (1,549,000) |  |   |
| Continuous Unrealized Loss Position, Fair Value                                                           | 788,036,000     |           | 788,036,000     |  |   |  | 158,949,000 |
| Amortized Cost                                                                                            | 1,321,239,000   |           | 1,321,239,000   |  |   |  | 1,169,613,000 |
| Estimated Fair Value                                                                                      | 1,291,063,000   |           | 1,291,063,000   |  |   |  | 1,171,303,000 |
| Number of banks and other financial institutions comprising the security                                  | 307             |           | 307             |  |   |  |   |
| Number of pooled securities representing senior tranches                                                  |            |           | 1               |  |   |  |   |
| Number Of Pooled Securities with No Senior Class                                                          |            |           | 4               |  |   |  |   |
| Amount of pooled issues that remained above investment grade                                              | 0               |           | 0               |  |   |  |   |
| Number of securities with no excess subordination                                                         | 7               |           | 7               |  |   |  |   |
| Number of Securities with Excess Subordination                                                            | 4               |           | 4               |  |   |  |   |
| Coupon rate                                                                                               |            |           | 7.00%           |  |   |  |   |
| Probability of default                                                                                    |            |           | 100.00%         |  |   |  |   |
| Projected recovery rate                                                                                   |            |           | 0.00%           |  |   |  |   |
| Probability percentage assigned to default bank subject to market indicators                              |            |           | 10.00%          |  |   |  |   |
| Probability percentage assigned to default bank                                                           |            |           | 100.00%         |  |   |  |   |
| Other than temporary impairment, credit losses recognized in earnings                                     | 1,326,000       | (436,000)      | 1,901,000       | [1] | 942,000 | [1] |   |
| Excess Subordination recognized related to paydowns                                                       | 1,000,000       |           |            |  |   |  |   |
| Minimum [Member]                                                                                          |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Subordinated tranches range                                                                               | 7.00%           |           | 7.00%           |  |   |  |   |
| Total principal amount of the respective securities                                                       | 5.00%           |           | 5.00%           |  |   |  |   |
| Excess subordination as a percentage of current performing collateral                                     |            |           | 0.00%           |  |   |  |   |
| Probabilities for performing collateral range                                                             |            |           | 0.33%           |  |   |  |   |
| Excess present value of future cash flows over our current book value                                     |            |           | 15.00%          |  |   |  |   |
| Maximum [Member]                                                                                          |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Subordinated tranches range                                                                               | 35.00%          |           | 35.00%          |  |   |  |   |
| Excess subordination as a percentage of current performing collateral                                     |            |           | 57.00%          |  |   |  |   |
| Probabilities for performing collateral range                                                             |            |           | 75.00%          |  |   |  |   |
| Excess present value of future cash flows over our current book value                                     |            |           | 165.00%         |  |   |  |   |
| Pooled Trust Preferred Collateralized Debt Obligations [Member]                                           |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Percent of unrealized losses                                                                              | 49.00%          |           | 49.00%          |  |   |  |   |
| Continuous Unrealized Loss Position, Fair Value                                                           | 22,637,000      |           | 22,637,000      |  |   |  | 23,316,000 |
| Amortized Cost                                                                                            | 48,824,000      |           | 48,824,000      |  |   |  | 51,866,000 |
| Estimated Fair Value                                                                                      | 25,457,000      |           | 25,457,000      |  |   |  | 23,373,000 |
| Trust preferred collateralized debt obligations [Member]                                                  |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Other than temporary impairment charges recognized                                                        |            |           | 0               |  | 0 |  |   |
| Percentage of Current Performing Collateral                                                               | 0.00%           |           | 0.00%           |  |   |  |   |
| Corporate Debt Securities [Member]                                                                        |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Amortized Cost                                                                                            | 6,697,000       |           | 6,697,000       |  |   |  | 6,703,000 |
| Estimated Fair Value                                                                                      | 7,001,000       |           | 7,001,000       |  |   |  | 6,991,000 |
| Pooled Trust Preferred Securities [Member]                                                                |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Other than temporary impairment charges recognized                                                        | 0               | 0              |            |  |   |  |   |
| Equity Securities [Member]                                                                                |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Other than temporary impairment charges recognized                                                        | 0               | 0              |            |  |   |  |   |
| Continuous Unrealized Loss Position, Fair Value                                                           | 0               | 0              | 0               |  | 0 |  |   |
| Amortized Cost                                                                                            | 1,420,000       |           | 1,420,000       |  |   |  | 1,859,000 |
| Estimated Fair Value                                                                                      | 1,420,000       |           | 1,420,000       |  |   |  | 1,975,000 |
| Asset Size and Coupon Rate 1 [Member]                                                                     |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Collateral issued by financial institutions                                                               | 15,000,000,000  |           | 15,000,000,000  |  |   |  |   |
| Prepayment rate                                                                                           |            |           | 100.00%         |  |   |  |   |
| Coupon rate                                                                                               |            |           | 7.00%           |  |   |  |   |
| Asset Size and Coupon Rate 2 [Member]                                                                     |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Coupon rate                                                                                               |            |           | 7.00%           |  |   |  |   |
| Fair Value Inputs , Prepayment Rate Year One and Two                                                      |            |           | 40.00%          |  |   |  |   |
| Fair Value Inputs , Prepayment Rate After Year Two                                                        |            |           | 2.00%           |  |   |  |   |
| Asset Size and Coupon Rate 3 [Member]                                                                     |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Prepayment rate                                                                                           |            |           | 5.00%           |  |   |  |   |
| Asset Size and Coupon Rate 3 [Member] | Minimum [Member]                                                  |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Collateral issued by financial institutions                                                               | 2,000,000,000   |           | 2,000,000,000   |  |   |  |   |
| Asset Size and Coupon Rate 3 [Member] | Maximum [Member]                                                  |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Collateral issued by financial institutions                                                               | 15,000,000,000  |           | 15,000,000,000  |  |   |  |   |
| Asset Size and Coupon Rate 4 [Member]                                                                     |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Collateral issued by financial institutions                                                               | $ 2,000,000,000 |           | $ 2,000,000,000 |  |   |  |   |
| Prepayment rate                                                                                           |            |           | 5.00%           |  |   |  |   |
| Coupon rate                                                                                               |            |           | 10.00%          |  |   |  |   |
| Asset Size and Coupon Rate 5 [Member]                                                                     |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Prepayment rate                                                                                           |            |           | 0.00%           |  |   |  |   |
| Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]    |            |           |            |  |   |  |   |
| **Schedule of Trading Securities and Other Trading Assets [Line Items]**                                  |            |           |            |  |   |  |   |
| Percent of unrealized losses                                                                              | 51.00%          |           | 51.00%          |  |   |  |   |
|                                                                                                           |
| [1]Represents the increase in cash flows recognized in interest income during the period.                 |

| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of pooled issues that remained above investment grade.
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| X                                                                                                                                                                                                                                                                                                |
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| - Definition
Available-for-sale, securities in unrealized loss, percentage.
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| X                                                                                                                                                                                                                                                                                                               |
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| - Definition
Excess present value of future cash flows over our current book value.
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| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Excess subordination as a percentage of current performing collateral.
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| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Excess Subordination recognized related to paydowns
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| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs, Collateral
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| X                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs, Coupon Rate
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| X                                                                                                                                                                                                                                                                          |
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| - Definition
Fair Value Inputs , Prepayment Rate After Year Two
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| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs , Prepayment Rate Year One and Two
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| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of banks and other financial institutions comprising the security.
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| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of pooled securities representing senior tranches.
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| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Pooled Securities with No Senior Class
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| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities with Excess Subordination
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| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities with No Excess Subordination
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| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of principal amount in pooled security securities.
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| - Definition
Percent of Unrealized Losses
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| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probabilities for performing collateral range.
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| X                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probability of default.
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| X                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probability percentage assigned to default bank.
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| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probability percentage assigned to default bank subject to market indicators.
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| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Projected recovery rate.
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| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Subordinated tranches range.
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to increases in cash flows expected to be collected and recognized over the remaining life of the security.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Before tax amount of other than temporary impairment (OTTI) on a debt security, categorized as available-for-sale, recognized in other comprehensive income (loss).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 19 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 16C -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 15D -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 130 -Paragraph 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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| - Definition
The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 7 -Footnote 2 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 15D -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 15C -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 16B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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