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Summary of Significant Accounting Policies - Stock-based compensation cost (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] |  |  |  |  |  |  |
Stock-based compensation cost | $ 503 | $ 327 | $ 850 | $ 988 | $ 1,189 | $ 2,813 | $ 5,020
Cost of revenue, excluding depreciation and amortization |  |  |  |  |  |  |
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] |  |  |  |  |  |  |
Stock-based compensation cost | 388 | 54 | 543 | 61 | 109 | 353 | 1,436
Sales and marketing |  |  |  |  |  |  |
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] |  |  |  |  |  |  |
Stock-based compensation cost | 37 | 28 | 98 | 31 | 60 | 658 | 1,442
General and administrative |  |  |  |  |  |  |
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] |  |  |  |  |  |  |
Stock-based compensation cost | 59 | 221 | 160 | 872 | 977 | 1,446 | 1,505
Research and development |  |  |  |  |  |  |
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items] |  |  |  |  |  |  |
Stock-based compensation cost | $ 19 | $ 24 | $ 49 | $ 24 | $ 43 | $ 356 | $ 637

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