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INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
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Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Due in one year or less | $ 201,210 |
Due after one through five years | 727,189 |
Due after five years through ten years | 954,210 |
Due after ten years | 1,442,839 |
Subtotal | 3,325,448 |
Amortized cost | 3,898,704 |
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Due in one year or less | 202,974 |
Due after one through five years | 758,445 |
Due after five years through ten years | 952,781 |
Due after ten years | 1,295,311 |
Subtotal | 3,209,511 |
Fair value | 3,791,391 |
Government agency mortgage-backed securities |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Without single maturity date | 537,317 |
Amortized cost | 537,317 | $ 506,540
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Without single maturity date | 544,986 |
Fair value | 544,986 | 522,638
Government agency collateralized mortgage obligations |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Without single maturity date | 35,939 |
Amortized cost | 35,939 | 42,476
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Without single maturity date | 36,894 |
Fair value | $ 36,894 | $ 43,577

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