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Stockholders’ Deficit - Common stock and preferred stock (Narrative) (Details) | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2017 shares | Sep. 30, 2025 $ / shares shares | Sep. 30, 2024 shares | Sep. 30, 2025 vote $ / shares shares | Sep. 30, 2024 shares | Dec. 31, 2024 shares
Class of Stock [Line Items] |  |  |  |  |  |
Preferred stock, shares authorized (in shares) |  | 240,000,000.0 |  | 240,000,000.0 |  |
Restricted Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Shares granted (in shares) |  |  |  | 14,600,000 |  |
Co-Founder Grant | Restricted Stock | Chief Executive Officer |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Shares granted (in shares) | 10,300,000 |  |  |  |  |
Class A common stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Votes per share | vote |  |  |  | 1 |  |
Common stock, shares authorized (in shares) |  | 2,400,000,000 |  | 2,400,000,000 |  |
Common stock, par value (in dollars per share) | $ / shares |  | $ 0.00001 |  | $ 0.00001 |  |
Common stock, shares issued (in shares) |  | 179,400,000 |  | 179,400,000 |  | 218,400,000
Common stock, shares outstanding (in shares) |  | 179,400,000 |  | 179,400,000 |  | 218,400,000
Class A common stock | Co-Founder Grant | Restricted Stock | Chief Executive Officer |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Shares granted (in shares) |  |  |  | 8,300,000 |  | 8,300,000
Class B common stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Votes per share | vote |  |  |  | 10 |  |
Common stock, shares authorized (in shares) |  | 475,000,000.0 |  | 475,000,000.0 |  |
Common stock, par value (in dollars per share) | $ / shares |  | $ 0.00001 |  | $ 0.00001 |  |
Shares converted in conversion (in shares) |  | 500,000 | 600,000 | 1,400,000 | 1,600,000 |
Common stock, shares issued (in shares) |  | 76,100,000 |  | 76,100,000 |  | 77,500,000
Common stock, shares outstanding (in shares) |  | 76,100,000 |  | 76,100,000 |  | 77,500,000
Class C common stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Votes per share | vote |  |  |  | 0 |  |
Common stock, shares authorized (in shares) |  | 800,000,000.0 |  | 800,000,000.0 |  |
Common stock, par value (in dollars per share) | $ / shares |  | $ 0.00001 |  | $ 0.00001 |  |
Common stock, shares issued (in shares) |  | 0 |  | 0 |  | 0
Common stock, shares outstanding (in shares) |  | 0 |  | 0 |  | 0

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