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Investment securities - Narrative (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 USD ($) security | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) security | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) security
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |
Interest receivable | $ 47,120,000 |  | $ 47,120,000 |  | $ 17,083,000
Equity securities, at fair value | 4,389,000 |  | 4,389,000 |  | 3,295,000
Trade date payable - securities |  |  | 1,214,000 | $ 3,671,000 | $ 0
Net (losses) gains on marketable equity securities | $ 59,000 | $ 55,000 | $ 94,000 | 151,000 |
Number of securities in securities portfolio | security | 553 |  | 553 |  | 365
Number of securities in securities portfolio, unrealized loss position | security | 33 |  | 33 |  | 58
Allowance for credit losses for investments | $ 0 |  | $ 0 |  |
Provision for credit losses on available for sale debt securities | 0 |  | 0 |  |
Other than temporary impairment |  | $ 0 |  | $ 0 |
Debt Securities |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |
Interest receivable | 4,753,000 |  | 4,753,000 |  | $ 2,843,000
Collateral Pledged |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |
Securities pledged | $ 607,304,000 |  | $ 607,304,000 |  | $ 373,674,000

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