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Investment Securities Available for Sale, at Fair Value - Narrative (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Schedule of Investments [Line Items] |  |  |  |
Contractual maturities (up to) |  | 40 years |  |
Weighted average life of available for sale securities portfolio |  | 7 years 1 month 6 days |  | 5 years
Allowance for credit loss |  | $ 0 |  |
Gross unrealized losses |  | (13,624,000) |  | $ (7,591,000)
Other-than-temporary impairment recorded in earnings | $ 0 |  | $ 0 |
Non-Agency RMBS |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Gross unrealized losses |  | (12,988,000) |  | (13,000)
CMBS |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Gross unrealized losses |  | $ (636,000) |  | $ (124,000)

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- Definition Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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