| **Impairment of Investment Securities - Cumulative Roll Forward of Credit Losses Recognized in Earnings for Debt Securities Held and Not Intended to be Sold (Detail) (USD $)In Thousands, unless otherwise specified**                                                             | 12 Months Ended |
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| **Impairment of Investment Securities Disclosure [Abstract]**                                                                                                                                                                                                                       |            |  |   |  |   |  |
| Balance, beginning                                                                                                                                                                                                                                                                  | $ 43,274        | [1] | $ 44,736 | [1] | $ 44,850 | [1] |
| Credit losses on debt securities for which other-than-temporary impairment was not previously recognized                                                                                                                                                                            | 0               |  | 0 |  | 0 |  |
| Additional credit losses on debt securities for which other-than- temporary impairment was previously recognized                                                                                                                                                                    | 0               |  | 0 |  | 0 |  |
| Increases in cash flows expected to be collected, recognized over the remaining life of the security                                                                                                                                                                                | (2,375)         | [2] | (1,462) | [2] | (114) | [2] |
| Reduction for debt securities called during the period                                                                                                                                                                                                                              | (13,356)        |  | 0 |  | 0 |  |
| Balance, ending                                                                                                                                                                                                                                                                     | $ 27,543        |  | $ 43,274 | [1] | $ 44,736 | [1] |
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| [1]The beginning balance represents credit related losses included in other-than-temporary impairment charges recognized on debt securities in prior periods.[2]Represents the increase in cash flows recognized either as principal payments or interest income during the period. |

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| - Definition
This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.
+ References
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| - Definition
This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized.
+ References
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| - Definition
The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).
+ 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
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 because of the sale of impaired securities during the period.
+ References
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