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Investment Securities Available for Sale (Narrative) (Details) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) securities | Jun. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) securities | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) securities
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
Investment securities gains, net | $ (1,550,000) | $ 69,400,000 | $ (3,731,000) | $ 76,594,000 | $ (5,502,000) |
Gross losses | 1,550,000 | 5,700,000 | 3,871,000 | 7,246,000 | 14,772,000 |
Gross realized gains on sales | $ 0 |  | 140,000 | $ 83,840,000 | 9,270,000 |
Investment securities in a loss position for less than twelve months | securities | 28 |  |  | 28 |  | 26
Investment securities in a loss position for twelve months or longer | securities | 0 |  |  | 0 |  | 5
Investment securities gains, net | $ (1,550,000) | $ 69,400,000 | $ (3,731,000) | $ 76,594,000 | $ (5,502,000) |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |  |  | 0 |  |
Debt Securities, Available-for-sale, Realized Loss - Quarter Two Settlement | 802,000 |  |  |  |  |
Collateral Pledged |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
Pledged to secure deposits | $ 2,340,000,000 |  |  | $ 2,340,000,000 |  | $ 1,710,000,000

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