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INVESTMENT SECURITIES - Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2022 | Dec. 31, 2021
Available-for-Sale Portfolio [Abstract] |  |
Amortized cost | $ 6,191,809 | $ 4,980,102
Gross unrealized gains | 9,766 | 65,387
Gross unrealized losses | 414,417 | 111,279
Fair value | 5,787,158 | 4,934,210
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 6,191,809 |
Debt Securities, Available-for-sale | 5,787,158 |
U.S. Treasury and Agency Securities |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 5,200,931 | 4,064,624
Gross unrealized gains | 4,301 | 39,997
Gross unrealized losses | 373,145 | 106,057
Debt Securities, Available-for-sale | 4,832,087 | 3,998,564
Obligations of State and Political Subdivisions |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 488,659 | 413,019
Gross unrealized gains | 4,220 | 17,326
Gross unrealized losses | 16,724 | 56
Debt Securities, Available-for-sale | 476,155 | 430,289
Government Agency Mortgage-Backed Securities |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 476,790 | 474,506
Gross unrealized gains | 1,213 | 7,615
Gross unrealized losses | 23,756 | 5,065
Debt Securities, Available-for-sale | 454,247 | 477,056
Corporate debt securities |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 8,000 | 8,000
Gross unrealized gains | 0 | 39
Gross unrealized losses | 503 | 77
Debt Securities, Available-for-sale | 7,497 | 7,962
Government Agency Collateralized Mortgage Obligations |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Amortized cost | 17,429 | 19,953
Gross unrealized gains | 32 | 410
Gross unrealized losses | 289 | 24
Debt Securities, Available-for-sale | 17,172 | 20,339
Federal Reserve Bank Stock [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Gross unrealized losses | 0 | 0
Other Equity Securities [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Gross unrealized losses | 0 | 0
Federal Home Loan Bank Common Stock [Member] |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |
Gross unrealized losses | $ 0 | $ 0

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