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INVESTMENT SECURITIES - Investment Securities Pledged as Collateral (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022 | Dec. 31, 2022
Investment Securities Pledged as Collateral |  |  |  |  |
Investment securities pledged to collateralize certain deposits and borrowings | $ 3,450,000 |  | $ 3,450,000 |  | $ 2,180,000
Gross realized losses | 0 | $ 0 | (52,329) | $ 0 |
Proceeds from sales of available-for-sale investment securities |  |  | 733,789 | $ 0 |
U.S. Treasury and agency securities |  |  |  |  |
Investment Securities Pledged as Collateral |  |  |  |  |
Book value of available-for-sale | 786,100 |  | 786,100 |  |
Gross realized losses |  |  | 52,300 |  |
Proceeds from sales of available-for-sale investment securities |  |  | 733,800 |  |
U.S. Treasury and agency securities | Securities Sold under Agreements to Repurchase |  |  |  |  |
Investment Securities Pledged as Collateral |  |  |  |  |
Investment securities pledged to collateralize certain deposits and borrowings | $ 597,100 |  | $ 597,100 |  | $ 466,900

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