| **INVESTMENT SECURITIES - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands** | Jun. 30, 2023 | Dec. 31, 2022 |
|------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | $ 1,921,167   | $ 2,273,347   |
| Gross Unrealized Gains                                                                                                 | 23            | 439           |
| Gross Unrealized Losses                                                                                                | 269,828       | 297,125       |
| Fair Value                                                                                                             | 1,651,362     | 1,976,661     |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 2,240,374     | 2,287,372     |
| Allowance for Credit Losses                                                                                            | 245           | 245           |
| Net Carrying Amount                                                                                                    | 2,240,129     | 2,287,127     |
| Gross Unrealized Gains                                                                                                 | 644           | 647           |
| Gross Unrealized Losses                                                                                                | 360,335       | 380,154       |
| Fair Value                                                                                                             | 1,880,683     | 1,907,865     |
| U.S. Treasury                                                                                                          |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 2,126         | 2,501         |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 35            | 42            |
| Fair Value                                                                                                             | 2,091         | 2,459         |
| U.S. Government-sponsored agency securities                                                                            |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 115,876       | 119,154       |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 17,522        | 17,192        |
| Fair Value                                                                                                             | 98,354        | 101,962       |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 381,915       | 392,246       |
| Allowance for Credit Losses                                                                                            | 0             | 0             |
| Net Carrying Amount                                                                                                    | 381,915       | 392,246       |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 69,466        | 69,147        |
| Fair Value                                                                                                             | 312,449       | 323,099       |
| State and municipal                                                                                                    |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 1,233,589     | 1,530,048     |
| Gross Unrealized Gains                                                                                                 | 23            | 438           |
| Gross Unrealized Losses                                                                                                | 154,269       | 178,726       |
| Fair Value                                                                                                             | 1,079,343     | 1,351,760     |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 1,111,355     | 1,117,552     |
| Allowance for Credit Losses                                                                                            | 245           | 245           |
| Net Carrying Amount                                                                                                    | 1,111,110     | 1,117,307     |
| Gross Unrealized Gains                                                                                                 | 644           | 647           |
| Gross Unrealized Losses                                                                                                | 180,637       | 197,064       |
| Fair Value                                                                                                             | 931,362       | 921,135       |
| U.S. Government-sponsored mortgage-backed securities                                                                   |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 556,602       | 608,630       |
| Gross Unrealized Gains                                                                                                 | 0             | 1             |
| Gross Unrealized Losses                                                                                                | 96,815        | 100,358       |
| Fair Value                                                                                                             | 459,787       | 508,273       |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 745,604       | 776,074       |
| Allowance for Credit Losses                                                                                            | 0             | 0             |
| Net Carrying Amount                                                                                                    | 745,604       | 776,074       |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 110,191       | 113,915       |
| Fair Value                                                                                                             | 635,413       | 662,159       |
| Corporate obligations                                                                                                  |          |          |
| **Available for sale securities**                                                                                      |          |          |
| Amortized Cost                                                                                                         | 12,974        | 13,014        |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 1,187         | 807           |
| Fair Value                                                                                                             | 11,787        | 12,207        |
| Foreign investment                                                                                                     |          |          |
| **Held to maturity securities**                                                                                        |          |          |
| Amortized Cost                                                                                                         | 1,500         | 1,500         |
| Allowance for Credit Losses                                                                                            | 0             | 0             |
| Net Carrying Amount                                                                                                    | 1,500         | 1,500         |
| Gross Unrealized Gains                                                                                                 | 0             | 0             |
| Gross Unrealized Losses                                                                                                | 41            | 28            |
| Fair Value                                                                                                             | $ 1,459       | $ 1,472       |

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