| **Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands** | Sep. 30, 2015 | Dec. 31, 2014 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | $ 1,045,850   | $ 1,317,318   |
| Gross Unrealized Gains                                                                                                                              | 17,069        | 16,921        |
| Gross Unrealized Losses                                                                                                                             | (12,186)      | (24,420)      |
| Securities available for sale, at fair value                                                                                                        | 1,050,733     | 1,309,819     |
| US Government Agencies Residential Mortgage-backed Securities [Member]                                                                              |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 20,902        | 23,344        |
| Gross Unrealized Gains                                                                                                                              | 2,379         | 2,595         |
| Gross Unrealized Losses                                                                                                                             | 0             | (3)           |
| Securities available for sale, at fair value                                                                                                        | 23,281        | 25,936        |
| Residential Mortgage Backed Securities [Member]                                                                                                     |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 816,179       | 947,635       |
| Gross Unrealized Gains                                                                                                                              | 12,819        | 13,076        |
| Gross Unrealized Losses                                                                                                                             | 4,846         | (9,830)       |
| Securities available for sale, at fair value                                                                                                        | 824,152       | 950,881       |
| Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]                                             |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 37            | 72            |
| Gross Unrealized Gains                                                                                                                              | 1             | 2             |
| Gross Unrealized Losses                                                                                                                             | 0             | 0             |
| Securities available for sale, at fair value                                                                                                        | 38            | 74            |
| Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]                                              |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 135,853       | 269,181       |
| Gross Unrealized Gains                                                                                                                              | 19            | 4             |
| Gross Unrealized Losses                                                                                                                             | 91            | (1,308)       |
| Securities available for sale, at fair value                                                                                                        | 135,781       | 267,877       |
| Obligations of States and Political Subdivisions [Member]                                                                                           |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 27,064        | 27,058        |
| Gross Unrealized Gains                                                                                                                              | 397           | 362           |
| Gross Unrealized Losses                                                                                                                             | 12            | (43)          |
| Securities available for sale, at fair value                                                                                                        | 27,449        | 27,377        |
| Corporate Debt Securities [Member]                                                                                                                  |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 1,895         | 6,682         |
| Gross Unrealized Gains                                                                                                                              | 438           | 573           |
| Gross Unrealized Losses                                                                                                                             | 0             | 0             |
| Securities available for sale, at fair value                                                                                                        | 2,333         | 7,255         |
| Pooled Trust Preferred Collateralized Debt Obligations [Member]                                                                                     |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 42,000        | 41,926        |
| Gross Unrealized Gains                                                                                                                              | 1,016         | 309           |
| Gross Unrealized Losses                                                                                                                             | (7,237)       | (13,236)      |
| Securities available for sale, at fair value                                                                                                        | 35,779        | 28,999        |
| Debt Securities [Member]                                                                                                                            |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 1,043,930     | 1,315,898     |
| Gross Unrealized Gains                                                                                                                              | 17,069        | 16,921        |
| Gross Unrealized Losses                                                                                                                             | (12,186)      | (24,420)      |
| Securities available for sale, at fair value                                                                                                        | 1,048,813     | 1,308,399     |
| Equity Securities [Member]                                                                                                                          |          |          |
| **Investment Securities [Line Items]**                                                                                                              |          |          |
| Amortized Cost                                                                                                                                      | 1,920         | 1,420         |
| Gross Unrealized Gains                                                                                                                              | 0             | 0             |
| Gross Unrealized Losses                                                                                                                             | 0             | 0             |
| Securities available for sale, at fair value                                                                                                        | $ 1,920       | $ 1,420       |

| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Securities [Line Items]
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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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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