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Stockholders' Equity - Co-Founder Grants and Award Modifications (Details) $ / shares in Units, shares in Millions, $ in Millions |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Mar. 19, 2020 USD ($) shares | Dec. 31, 2017 USD ($) tranche $ / shares shares | Mar. 31, 2020 USD ($) | Jun. 30, 2019 USD ($) | Jun. 30, 2020 USD ($) $ / shares shares | Jun. 30, 2019 USD ($) | Dec. 31, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Weighted average grant date fair value (in dollars per share) | $ / shares |  |  |  |  | $ 5.55 |  |
Reversal of stock based compensation expense | $ | $ 23.8 |  | $ 23.8 |  |  |  |
Reversal Of Stock Based Compensation Previously Recognized | $ | $ 21.5 |  |  |  |  |  |
Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares granted (in shares) |  |  |  |  | 20.8 |  |
Shares forfeited (in shares) |  |  |  |  | 5.8 |  |
Co-Founder Grants | Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares granted (in shares) |  | 14.7 |  |  | 10.3 |  | 14.7
Expiration period |  | 10 years |  |  |  |  |
Number of tranches | tranche |  | 9 |  |  |  |  |
Weighted average grant date fair value (in dollars per share) | $ / shares |  | $ 10.60 |  |  |  |  |
Unamortized stock-based compensation, requisite service period |  | 5 years 2 months 12 days |  |  |  |  |
Stock-based compensation related to the Co-Founder Grants | $ |  |  | $ 6.1 | $ 8.7 | $ 12.2 | $ 17.3 |
Remaining unamortized stock-based compensation | $ |  | $ 156.2 |  |  | $ 46.8 |  |
Shares forfeited (in shares) | 4.4 |  |  |  |  |  |
Co-Founder Grants | Restricted Stock | Minimum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Unamortized stock-based compensation, requisite service period |  | 2 years 10 months 24 days |  |  |  |  |
Co-Founder Grants | Restricted Stock | Maximum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Unamortized stock-based compensation, requisite service period |  | 6 years 10 months 24 days |  |  |  |  |
Co-Founder Grants | Restricted Stock | Tranche One |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Vesting period |  | 4 years |  |  |  |  |
Co-Founder Grants | Restricted Stock | Tranche One | Maximum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Percentage vested maximum |  | 20.00% |  |  |  |  |
Co-Founder Grants | Chief Executive Officer | Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares granted (in shares) |  | 10.3 |  |  |  |  |
Co-Founder Grants | Director | Restricted Stock |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares granted (in shares) |  | 4.4 |  |  |  |  |
Shares forfeited (in shares) | 4.4 |  |  |  |  |  |

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