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INVESTMENT SECURITIES - Amortized cost and estimated fair value of debt securities by contractual maturity (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Due in one year or less | $ 17,031 |
Due after one through five years | 1,473,904 |
Due after five years through ten years | 685,913 |
Due after ten years | 716,597 |
Subtotal | 2,893,445 |
Amortized Cost | 3,314,815 | $ 4,675,474
Available-for-Sale, Fair Value |  |
Due in one year or less | 16,799 |
Due after one through five years | 1,295,411 |
Due after five years through ten years | 587,185 |
Due after ten years | 521,683 |
Subtotal | 2,421,078 |
Fair Value | 2,767,636 | 4,151,851
Held-to-Maturity, Amortized Cost |  |
Due in one year or less | 0 |
Due after one through five years | 0 |
Due after five years through ten years | 549,555 |
Due after ten years | 552,124 |
Subtotal | 1,101,679 |
Government agency mortgage-backed securities | 1,125,540 | 1,079,695
Amortized Cost | 1,125,540 |
Held-to-Maturity, Fair Value |  |
Due in one year or less | 0 |
Due after one through five years | 0 |
Due after five years through ten years | 506,275 |
Due after ten years | 461,313 |
Subtotal | 967,588 |
Fair Value | 990,660 | 1,034,795
Government agency mortgage-backed securities |  |
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Amortized Cost | 411,083 | 444,689
Available-for-Sale, Fair Value |  |
Fair Value | 337,071 | 384,633
Held-to-Maturity, Amortized Cost |  |
Government agency mortgage-backed securities | 23,861 |
Held-to-Maturity, Fair Value |  |
Fair Value | 23,072 | 0
Government agency collateralized mortgage obligations |  |
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Amortized Cost | 10,287 | 13,121
Available-for-Sale, Fair Value |  |
Fair Value | 9,487 | $ 12,270
Held-to-Maturity, Amortized Cost |  |
Amortized Cost | 0 |
Held-to-Maturity, Fair Value |  |
Fair Value | $ 0 |

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