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Commitments and Contingencies - Additional Information (Details) |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) RenewalOption | Nov. 21, 2017 USD ($) Facility | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) RenewalOption | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Estimate of milestone payments | $ 15,500,000 |  |  | $ 15,500,000 |  |
Operating leases, expiry date |  |  |  | 2023-04 |  |
Operating leases, number of successive renewal options | RenewalOption | 2 |  |  | 2 |  |
Operating leases, extended lease term |  |  |  | 5 years |  |
Lease agreement term |  |  | 63 months |  |  |
Lease commencement date |  |  | Nov. 14, 2017 |  |  |
Total lease payments | $ 18,946,000 |  | $ 900,000 | $ 18,946,000 |  |
Lease payments term |  |  | 2023-02 |  |  |
Operating leases, total rental expense |  |  |  | $ 2,400,000 | $ 3,800,000 | $ 3,700,000
Operating leases, number of portions of leased facilities | Facility |  | 1 |  |  |  |
Operating leases, number of leased facilities | Facility |  | 3 |  |  |  |
Operating leases, sublease commencement date |  | Dec. 26, 2017 |  |  |  |
Operating leases sublease income receivable |  | $ 5,100,000 |  |  |  |
Operating leases sublease expiry date |  | 2023-04 |  |  |  |
Sublease tenant improvement allowance |  | $ 800,000 |  |  |  |
Standby Letter of Credit [Member] |  |  |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |  |  |
Letter of credit amount |  | $ 1,000,000 |  |  |  |
Funds drawn from letter of credit | $ 0 |  |  |  |  |

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