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CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands | Total | Cumulative Effect, Period of Adoption, Adjustment | Common Stock | Treasury Stock | Additional Paid-In Capital | Accumulated Deficit | Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment | Accumulated Other Comprehensive Loss
--- | --- | --- | --- | --- | --- | --- | --- | ---
Balance (in shares) at Dec. 31, 2017 |  |  | 36,859,077 | 0 |  |  |  |
Balance at Dec. 31, 2017 | $ 299,238 | $ (6,478) | $ 369 | $ 0 | $ 611,270 | $ (312,340) | $ (6,478) | $ (61)
Share-based compensation | 16,520 |  |  |  | 16,520 |  |  |
Issuance of common stock, net of offering costs (in shares) |  |  | 5,175,000 |  |  |  |  |
Issuance of common stock, net of offering costs | 103,259 |  | $ 52 |  | 103,207 |  |  |
Stock plan related activity (in shares) |  |  | 319,224 | 11,070 |  |  |  |
Stock plan related activity | 1,733 |  | $ 3 | $ (260) | 1,990 |  |  |
Retirement of treasury stock (in shares) |  |  |  | (11,070) |  |  |  |
Retirement of treasury stock | 0 |  |  | $ 260 | (260) |  |  |
Unrealized gain on investments | 58 |  |  |  |  |  |  | 58
Net loss | (171,453) |  |  |  |  | (171,453) |  |
Balance (in shares) at Dec. 31, 2018 |  |  | 42,353,301 | 0 |  |  |  |
Balance at Dec. 31, 2018 | 242,877 |  | $ 424 | $ 0 | 732,727 | (490,271) |  | (3)
Share-based compensation | 19,571 |  |  |  | 19,571 |  |  |
Issuance of common stock, net of offering costs (in shares) |  |  | 6,325,000 |  |  |  |  |
Issuance of common stock, net of offering costs | 118,657 |  | $ 63 |  | 118,594 |  |  |
Stock plan related activity (in shares) |  |  | 280,462 |  |  |  |  |
Stock plan related activity | 1,315 |  | $ 3 |  | 1,312 |  |  |
Unrealized gain on investments | 19 |  |  |  |  |  |  | 19
Net loss | (151,811) |  |  |  |  | (151,811) |  |
Balance (in shares) at Dec. 31, 2019 |  |  | 48,958,763 | 0 |  |  |  |
Balance at Dec. 31, 2019 | 230,628 |  | $ 490 | $ 0 | 872,204 | (642,082) |  | 16
Share-based compensation | 20,676 |  |  |  | 20,676 |  |  |
Issuance of common stock, net of offering costs (in shares) |  |  | 6,612,815 |  |  |  |  |
Issuance of common stock, net of offering costs | 170,456 |  | $ 66 |  | 170,390 |  |  |
Stock plan related activity (in shares) |  |  | 673,193 | 74,632 |  |  |  |
Stock plan related activity | 3,886 |  | $ 6 | $ (2,012) | 5,892 |  |  |
Retirement of treasury stock (in shares) |  |  |  | (74,632) |  |  |  |
Retirement of treasury stock | 0 |  |  | $ 2,012 | (2,012) |  |  |
Unrealized gain on investments | (23) |  |  |  |  |  |  | (23)
Net loss | (129,739) |  |  |  |  | (129,739) |  |
Balance (in shares) at Dec. 31, 2020 |  |  | 56,244,771 | 0 |  |  |  |
Balance at Dec. 31, 2020 | $ 295,884 |  | $ 562 | $ 0 | $ 1,067,150 | $ (771,821) |  | $ (7)

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