REDEMPTION SETTLEMENT ASSETS, RESTRICTED - Components of redemption settlement assets (Details) - USD ($) $ in Thousands |
Sep. 30, 2021 |
Dec. 31, 2020 |
|---|---|---|
| Schedule Of Redemption Settlement Assets [Line Items] | ||
| Redemption settlement assets, Fair Value, Total | $ 733,952 | $ 693,461 |
| Restricted cash | ||
| Schedule Of Redemption Settlement Assets [Line Items] | ||
| Redemption settlement assets, Fair Value, Total | 58,196 | 55,427 |
| Mutual funds | ||
| Schedule Of Redemption Settlement Assets [Line Items] | ||
| Redemption settlement assets, Fair Value, Total | 26,220 | 26,850 |
| Corporate bonds | ||
| Schedule Of Redemption Settlement Assets [Line Items] | ||
| Redemption settlement assets, Fair Value, Total | $ 649,536 | $ 611,184 |