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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
May 03, 2017 | Apr. 26, 2017 | Jul. 31, 2015 | Jan. 31, 2015 | Feb. 28, 2014 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  |  |  |  |  | 125,000,000 | 125,000,000 |
Common stock, par value (in dollars per share) |  |  |  |  |  | $ 0.01 | $ 0.01 |
Number of shares issued or sold (in shares) |  |  | 3,525,000 |  | 1,800,000 |  |  |
Price per share (in dollars per share) |  |  | $ 37.00 |  | $ 36.50 |  |  |
Additional shares sold under over-allotments option to underwriter (in shares) |  |  | 528,750 |  | 450,000 |  |  |
Proceeds from sale of shares |  |  | $ 141,000 |  | $ 76,700 |  |  |
Proceeds from issuance of common stock, net of offering costs |  |  |  |  |  | $ 34,244 | $ 0 | $ 203,467
Johnson & Johnson Innovation JJDC, Inc, |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Sale of common stock (in shares) |  |  |  | 1,923,077 |  |  |  |
Sale of common stock (in dollars per share) |  |  |  | $ 39 |  |  |  |
Proceeds of stock sale |  |  |  | $ 75,000 |  |  |  |
Undesignated Preferred Stock |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Undesignated preferred stock, shares authorized (in shares) |  |  |  |  |  | 5,000,000 |  |
Undesignated preferred stock, par value (in dollars per share) |  |  |  |  |  | $ 0.01 |  |
Undesignated preferred stock, shares issued (in shares) |  |  |  |  |  | 0 | 0 |
Undesignated preferred stock, shares outstanding (in shares) |  |  |  |  |  | 0 | 0 |
Registered Direct Offering |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Number of shares issued or sold (in shares) |  | 1,100,000 |  |  |  |  |  |
Common stock purchase price (in dollars per share) |  | $ 21.50 |  |  |  |  |  |
Proceeds from issuance of common stock, net of offering costs |  | $ 23,700 |  |  |  |  |  |
At The Market Offering |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Number of shares issued or sold (in shares) |  |  |  |  |  | 599,284 |  |
Proceeds of stock sale |  |  |  |  |  | $ 10,800 |  |
Common stock maximum amount available for issuance | $ 75,000 |  |  |  |  |  |  |

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