Investment securities - Additional Information (Details) - USD ($) $ in Millions |
Sep. 30, 2021 |
Dec. 31, 2020 |
Dec. 31, 2019 |
|---|---|---|---|
| Investments, Debt and Equity Securities [Abstract] | |||
| Deposits with various states and regulatory bodies | $ 11.2 | $ 7.5 | $ 3.7 |