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Restructuring - Narrative (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
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Restructuring | $ 6,162 | $ 0 | $ 6,162 | $ 18,089 | $ 18,089 |
Impairment of right-of-use assets |  |  | 5,731 | $ 5,411 |  |
Provision balance | 10,015 |  | 10,015 |  | $ 10,748 | $ 1,637
Office consolidation |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Provision balance | 8,900 |  | 8,900 |  |  |
Other liabilities | Office consolidation |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Provision balance | 4,800 |  | 4,800 |  |  |
Other liabilities | Employee severance |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Provision balance | 1,100 |  | 1,100 |  |  |
Non-current operating lease liabilities | Office consolidation |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Provision balance | $ 4,100 |  | $ 4,100 |  |  |

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