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INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | $ 1,443,800 | $ 1,078,474
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | 1,455,210 | 1,115,813
Obligations of U.S. government-sponsored enterprises |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | 8,585 |
Unrealized Gains | 1 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 134 |
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | 8,452 |
Obligations of states and political subdivisions |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | 112,688 | 119,608
Unrealized Gains | 5,267 | 7,627
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 202 | 115
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | 117,753 | 127,120
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | 915,317 | 547,396
Unrealized Gains | 11,286 | 19,796
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 6,611 | 574
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | 919,992 | 566,618
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | 391,128 | 399,937
Unrealized Gains | 4,595 | 10,652
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2,896 | 135
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | 392,827 | 410,454
Subordinated corporate bonds |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | 16,082 | 11,533
Unrealized Gains | 205 | 186
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 101 | 98
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | 16,186 | 11,621
Total AFS debt securities |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized Cost, Total | 1,443,800 | 1,078,474
Unrealized Gains | 21,354 | 38,261
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 9,944 | 922
Available-for-sale securities, at fair value (book value of $1,443,800 and $1,078,474, respectively) | $ 1,455,210 | $ 1,115,813

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