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INVESTMENT SECURITIES - Amortized cost and estimated fair value of debt securities by contractual maturity (Details) - USD ($) $ in Thousands | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Due in one year or less | $ 170,995 |
Due after one through five years | 1,013,648 |
Due after five years through ten years | 1,100,527 |
Due after ten years | 781,766 |
Subtotal | 3,066,936 |
Amortized cost | 3,500,726 | $ 4,675,474
Available-for-Sale, Fair Value |  |
Due in one year or less | 170,364 |
Due after one through five years | 909,908 |
Due after five years through ten years | 972,563 |
Due after ten years | 636,070 |
Subtotal | 2,688,905 |
Fair value | 3,064,870 | 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 | 546,275 |
Due after ten years | 547,992 |
Government agency mortgage-backed securities | 1,104,020 | 1,079,695
Subtotal | 1,094,267 |
Total - Amortized Cost | 1,104,020 |
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 | 529,303 |
Due after ten years | 524,510 |
Subtotal | 1,053,813 |
Total - Fair Value | 1,063,444 | 1,034,795
Government agency mortgage-backed securities |  |
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Amortized cost | 422,521 | 444,689
Available-for-Sale, Fair Value |  |
Fair value | 365,506 | 384,633
Held-to-Maturity, Amortized Cost |  |
Government agency mortgage-backed securities | 9,753 |
Held-to-Maturity, Fair Value |  |
Total - Fair Value | 9,631 | 0
Government agency collateralized mortgage obligations |  |
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Amortized cost | 11,269 | 13,121
Available-for-Sale, Fair Value |  |
Fair value | 10,459 | 12,270
Held-to-Maturity, Amortized Cost |  |
Total - Amortized Cost | 0 |
Held-to-Maturity, Fair Value |  |
Total - Fair Value | 0 |
U.S. Treasury Securities |  |
Available-for-Sale, Debt Maturities, Amortized Cost |  |
Amortized cost | 2,529,014 | 3,660,546
Available-for-Sale, Fair Value |  |
Fair value | 2,189,845 | 3,243,537
Held-to-Maturity, Fair Value |  |
Total - Fair Value | $ 1,053,813 | $ 1,034,795

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