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Investment Securities (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Held-to-maturity securities |  |
Amortized cost | $ 3,400,000 | $ 2,400,000
Estimated fair value | 3,400,000 | 2,400,000
Carrying Value | 3,400,000 | 2,400,000
Available-for-sale securities |  |
Amortized cost | 3,054,289,000 | 3,376,070,000
Gross unrealized gains | 35,832,000 |
Gross unrealized losses | (9,353,000) |
Available for sale debt securities | 3,080,768,000 | 3,378,923,000
Total Amortized cost | 3,054,289,000 | 3,376,070,000
Total Gross unrealized gains |  | 19,467,000
Total Gross unrealized losses |  | 16,614,000
Available for sale debt securities and equity securities | 3,080,768,000 |
Estimate of Fair Value Measurement [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities and equity securities | 3,080,768,000 | 3,378,923,000
Reported Value Measurement [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities and equity securities | 3,080,768,000 | 3,378,923,000
Collateralized Mortgage Backed Securities [Member] |  |
Available-for-sale securities |  |
Amortized cost | 3,006,592,000 | 3,285,623,000
Gross unrealized gains | 32,701,000 | 16,534,000
Gross unrealized losses | (9,339,000) | (16,609,000)
Collateralized Mortgage Backed Securities [Member] | Estimate of Fair Value Measurement [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities | 3,029,954,000 | 3,285,548,000
Collateralized Mortgage Backed Securities [Member] | Reported Value Measurement [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities | 3,029,954,000 | 3,285,548,000
US Government Corporations and Agencies Securities [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities | 371,407,000 | 571,247,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities | 2,658,547,000 | 2,714,301
US States and Political Subdivisions Debt Securities [Member] |  |
Available-for-sale securities |  |
Amortized cost | 47,697,000 | 90,447,000
Gross unrealized gains | 3,131,000 | 2,933,000
Gross unrealized losses | (14,000) | (5,000)
US States and Political Subdivisions Debt Securities [Member] | Estimate of Fair Value Measurement [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities | 50,814,000 | 93,375,000
US States and Political Subdivisions Debt Securities [Member] | Reported Value Measurement [Member] |  |
Available-for-sale securities |  |
Available for sale debt securities | 50,814,000 | 93,375,000
Other Debt Obligations [Member] |  |
Held-to-maturity securities |  |
Amortized cost | 3,400,000 | 2,400,000
Estimated fair value | 3,400,000 | 2,400,000
Carrying Value | $ 3,400,000 | $ 2,400,000

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