| **Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $)In Thousands, unless otherwise specified** | 9 Months Ended | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |           |            |
| Amortized Cost - Available for sale securities                                                                                                | $ 610,181      | $ 498,104       |
| Gross Unrealized Gains - Available for sale securities                                                                                        | 11,137         | 21,184          |
| Gross Unrealized Losses - Available for sale securities                                                                                       | 14,977         | 5,945           |
| Investment securities available for sale                                                                                                      | 606,341        | 513,343         |
| Amortized Cost - Held to maturity securities                                                                                                  | 307,794        | 361,020         |
| Gross Unrealized Gains - Held to maturity securities                                                                                          | 7,098          | 17,170          |
| Gross Unrealized Losses - Held to maturity securities                                                                                         | 509            | 16              |
| Investment securities held to maturity - fair value                                                                                           | 314,383        | 378,174         |
| Amortized Cost - Total Investment Securities                                                                                                  | 917,975        | 859,124         |
| Gross Unrealized Gains - Total Investment Securities                                                                                          | 18,235         | 38,354          |
| Gross Unrealized Losses - Total Investment Securities                                                                                         | 15,486         | 5,961           |
| Fair Value - Total Investment Securities                                                                                                      | 920,724        | 891,517         |
| U.S. Government-sponsored agency securities                                                                                                   |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |           |            |
| Amortized Cost - Available for sale securities                                                                                                | 3,791          | 4,475           |
| Gross Unrealized Gains - Available for sale securities                                                                                        | 18             | 165             |
| Gross Unrealized Losses - Available for sale securities                                                                                       | 9              |            |
| Investment securities available for sale                                                                                                      | 3,800          | 4,640           |
| State and municipal                                                                                                                           |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |           |            |
| Amortized Cost - Available for sale securities                                                                                                | 200,662        | 148,187         |
| Gross Unrealized Gains - Available for sale securities                                                                                        | 4,983          | 10,025          |
| Gross Unrealized Losses - Available for sale securities                                                                                       | 3,463          | 18              |
| Investment securities available for sale                                                                                                      | 202,182        | 158,194         |
| Amortized Cost - Held to maturity securities                                                                                                  | 115,796        | 117,227         |
| Gross Unrealized Gains - Held to maturity securities                                                                                          | 498            | 5,489           |
| Gross Unrealized Losses - Held to maturity securities                                                                                         | 6              | 1               |
| Investment securities held to maturity - fair value                                                                                           | 116,288        | 122,715         |
| U.S. Government-sponsored mortgage-backed securities                                                                                          |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |           |            |
| Amortized Cost - Available for sale securities                                                                                                | 397,687        | 337,631         |
| Gross Unrealized Gains - Available for sale securities                                                                                        | 6,136          | 10,994          |
| Gross Unrealized Losses - Available for sale securities                                                                                       | 5,654          | 46              |
| Investment securities available for sale                                                                                                      | 398,169        | 348,579         |
| Amortized Cost - Held to maturity securities                                                                                                  | 191,998        | 243,793         |
| Gross Unrealized Gains - Held to maturity securities                                                                                          | 6,600          | 11,681          |
| Gross Unrealized Losses - Held to maturity securities                                                                                         | 503            | 15              |
| Investment securities held to maturity - fair value                                                                                           | 198,095        | 255,459         |
| Corporate obligations                                                                                                                         |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |           |            |
| Amortized Cost - Available for sale securities                                                                                                | 6,335          | 6,105           |
| Gross Unrealized Losses - Available for sale securities                                                                                       | 5,851          | 5,881           |
| Investment securities available for sale                                                                                                      | 484            | 224             |
| Equity securities                                                                                                                             |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                                   |           |            |
| Amortized Cost - Available for sale securities                                                                                                | 1,706          | 1,706           |
| Investment securities available for sale                                                                                                      | $ 1,706        | $ 1,706         |

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.
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Fair value of investments in debt securities classified as held-to-maturity.
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Amount of unrecognized gain before loss on investments in held-to-maturity securities.
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