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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions |  | 1 Months Ended | 3 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 21, 2018 | Oct. 31, 2019 | Aug. 31, 2018 | Sep. 30, 2017 | Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2019 | Aug. 06, 2018
Stock transactions |  |  |  |  |  |  |  |  |
Par value (per share) |  |  |  |  | $ 0.00001 |  |  | $ 0.00001 |
2017 Form S-3 |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Maximum aggregate offering price |  |  |  | $ 200.0 |  |  |  |  |
2019 Form S-3 |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Maximum aggregate offering price |  | $ 250.0 |  |  | $ 0.0 |  |  |  |
2017 Sales Agreement |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Common stock sold (in shares) |  |  |  |  |  | 0 |  |  |
Common Stock | 2017 Form S-3 |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Common stock available for sale |  |  |  |  | $ 72.0 |  |  |  |
Common Stock | Private Placement |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Number of warrants exercised |  |  |  |  | 0 |  |  |  |
At-the-market offering (ATM) | 2019 Form S-3 |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Maximum aggregate offering price |  | $ 75.0 |  |  |  |  |  |  |
At-the-market offering (ATM) | 2017 Sales Agreement |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Maximum aggregate offering price |  |  |  | $ 50.0 |  |  |  |  |
Investors | Common Stock |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Price (in dollars per share) |  |  |  |  |  |  |  |  | $ 8.00
Common stock sold (in shares) |  |  | 2,750,000 |  |  |  |  |  |
Aggregate gross proceeds |  |  |  |  |  |  |  |  | $ 21.7
Investors | Common Stock | Private Placement |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Warrants to purchase common stock, number of shares |  |  |  |  |  |  |  |  | 2,750,000
Exchange Agreement | Exchanging Stockholders |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Warrants to purchase common stock, number of shares | 1,000,000 |  |  |  |  |  |  |  |
Common stock exchanged for pre-funded warrants |  |  |  |  |  |  | 1,000,000 |  |
Par value (per share) | $ 0.00001 |  |  |  |  |  |  |  |
Exercise Price (per share) | $ 0.00001 |  |  |  |  |  |  |  |
Duration of warrants from date of issuance (in years) | 10 years |  |  |  |  |  |  |  |
Equity method investment, ownership percentage | 9.99% |  |  |  |  |  |  |  |
Exchange Agreement | Exchanging Stockholders | Common Stock |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Number of warrants exercised |  |  |  |  | 600,000 |  |  |  |
Issuance of common stock upon exercise of Exchange Warrants (in shares) |  |  |  |  | 599,997 |  |  |  |
Exchange warrants unexercised |  |  |  |  | 400,000 |  |  |  |
Group one | Common Stock | Private Placement |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Number of shares to be converted for each warrant |  |  |  |  |  |  |  |  | 1,375,000
Exercise Price (per share) |  |  |  |  |  |  |  |  | $ 10.00
Group two | Common Stock | Private Placement |  |  |  |  |  |  |  |  |
Stock transactions |  |  |  |  |  |  |  |  |
Number of shares to be converted for each warrant |  |  |  |  |  |  |  |  | 1,375,000
Exercise Price (per share) |  |  |  |  |  |  |  |  | $ 15.00

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