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SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands |  | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jul. 30, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Jul. 26, 2021 | Dec. 31, 2020
Subsequent Event [Line Items] |  |  |  |  |
Common stock issued (in shares) |  | $ 1 |  |  | $ 1
Deferred offering costs |  | 2,627 |  |  | $ 0
Deferred offering costs incurred but not yet paid |  | $ 1,076 | $ 0 |  |
Subsequent Event |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Proceeds from Issuance of initial public offering | $ 455,523 |  |  |  |
Deferred Offering Costs Reclassified to Stockholders Equity | $ 4,000 |  |  |  |
Common Class A | Subsequent Event | 2021 Plan |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Common stock reserved for future issuance (in shares) |  |  |  | 7,832 |
Common Class A | Subsequent Event | Employee Stock Purchase Plan |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Common stock reserved for future issuance (in shares) |  |  |  | 1,119 |
Common Class A | Subsequent Event | IPO |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 5,872 |  |  |  |
Sale of stock, price per share (in usd per share) | $ 102.00 |  |  |  |
Common Class A | Subsequent Event | IPO | Duolingo |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 4,466 |  |  |  |
Common Class A | Subsequent Event | IPO | Stockholders |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 1,406 |  |  |  |
Common Class A | Subsequent Event | Over-Allotment Option |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 766 |  |  |  |
Common Class B | Subsequent Event |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Convertible preferred stock, shares issued upon conversion (in shares) | 19,074 |  |  |  |
Common stock issued (in shares) | $ 182,609 |  |  |  |

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