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INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Due in one year or less | $ 198,566 |
Due after one through five years | 768,349 |
Due after five years through ten years | 1,018,820 |
Due after ten years | 1,504,697 |
Subtotal | 3,490,432 |
Amortized cost | 4,039,978 |
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Due in one year or less | 199,557 |
Due after one through five years | 796,228 |
Due after five years through ten years | 1,037,749 |
Due after ten years | 1,420,784 |
Subtotal | 3,454,318 |
Fair value | 4,012,739 |
Government agency mortgage-backed securities |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Without single maturity date | 521,056 |
Amortized cost | 521,056 | $ 506,540
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Without single maturity date | 529,139 |
Fair value | 529,139 | 522,638
Government agency collateralized mortgage obligations |  |
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] |  |
Without single maturity date | 28,490 |
Amortized cost | 28,490 | 42,476
Available-for-Sale, Debt Maturities, Fair Value [Abstract] |  |
Without single maturity date | 29,282 |
Fair value | $ 29,282 | $ 43,577

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27198-111563 + Details
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