| **Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (USD $)In Thousands, unless otherwise specified**                                                                                                                                                                                    | Sep. 30, 2013 | Dec. 31, 2012 | Sep. 30, 2013Mortgage Backed Securities [Member] | Sep. 30, 2012Mortgage Backed Securities [Member] | Sep. 30, 2013US Government Agencies Residential Mortgage-backed Securities [Member] | Dec. 31, 2012US Government Agencies Residential Mortgage-backed Securities [Member] | Sep. 30, 2013Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|--------------------------------------------------|--------------------------------------------------|-------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------|
| **Investment Securities [Line Items]**                                                                                                                                                                                                                                                                                                                |          |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due within one year, Amortized Cost                                                                                                                                                                                                                                                                                                                   | $ 28,081      |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due after one but within five years, Amortized Cost                                                                                                                                                                                                                                                                                                   | 239,969       |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due after five but within ten years, Amortized Cost                                                                                                                                                                                                                                                                                                   | 0             |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due after ten years, Amortized Cost                                                                                                                                                                                                                                                                                                                   | 55,521        |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Debt securities gross, Amortized Cost                                                                                                                                                                                                                                                                                                                 | 323,571       |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Corporate/Mortgage-Backed Securities, Amortized cost                                                                                                                                                                                                                                                                                                  | 1,321,239     | 1,169,613     | 996,248                                          | [1]                                              | 990,800                                                                             | [1]                                                                                 | 23,787                                                                                          | 27,883 | 972,500 |
| Debt Securities, Amortized Cost                                                                                                                                                                                                                                                                                                                       | 1,319,819     |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due within one year, Estimated Fair Value                                                                                                                                                                                                                                                                                                             | 28,215        |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due after one but within five years, Estimated Fair Value                                                                                                                                                                                                                                                                                             | 238,170       |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due after five but within ten years, Estimated Fair Value                                                                                                                                                                                                                                                                                             | 0             |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Due after ten years, Estimated Fair Value                                                                                                                                                                                                                                                                                                             | 32,458        |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Debt securities gross, Estimated Fair Value                                                                                                                                                                                                                                                                                                           | 298,843       |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
| Total Debt Securities, Estimated Fair Value                                                                                                                                                                                                                                                                                                           | $ 1,289,643   |          |                                             |                                                  |                                                                                |                                                                                     |                                                                                            |   |   |
|                                                                                                                                                                                                                                                                                                                                                       |
| [1]Mortgage Backed Securities include an amortized cost of $23.8 million and a fair value of $26.5 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $972.5 million and a fair value of $964.3 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac. |

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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.
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| - Definition
Amount of available-for-sale debt securities at cost, net of adjustments, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.
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| - Definition
Amount of available-for-sale debt securities at fair value maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year.
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| - Definition
Amount of available-for-sale debt securities at cost, net of adjustments, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.
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| - Definition
Amount of available-for-sale debt securities at fair value maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year.
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| - Definition
Amount of available-for-sale debt securities at cost, net of adjustments, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.
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| - Definition
Amount of available-for-sale debt securities at fair value maturing after the tenth fiscal year following the latest fiscal year.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of available-for-sale debt securities at cost, net of adjustments, which include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.
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| - Definition
Amount of available-for-sale debt securities at cost, net of adjustments, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.
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| - Definition
Amount of available-for-sale debt securities at fair value maturing in the next fiscal year following the latest fiscal year.
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