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Restructuring Charges - Additional Information (Details) | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2019 USD ($) Facility | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 Employee | Jan. 31, 2019 Facility | Dec. 31, 2018 USD ($) | Oct. 31, 2018 Facility | Apr. 14, 2018 Facility | Nov. 21, 2017 Facility | Dec. 31, 2016 Facility
Restructuring Cost And Reserve [Line Items] |  |  |  |  |  |  |  |  |  |
Number of employees terminated | Employee |  |  |  | 62 |  |  |  |  |  |
Operating leases, number of leased facilities | Facility |  |  |  |  | 3 |  | 3 | 3 | 3 |
Lease-related Costs [Member] |  |  |  |  |  |  |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |  |  |  |  |  |  |
Restructuring charges (credit) | $ 0 |  | $ 0 |  |  |  |  |  |  |
Lease-related Costs [Member] | Accrued and Other Liabilities [Member] |  |  |  |  |  |  |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |  |  |  |  |  |  |
Combined lease-related liabilities, current portion |  |  |  |  |  | $ 1,400,000 |  |  |  |
Lease-related Costs [Member] | Other Liabilities Non-current [Member] |  |  |  |  |  |  |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |  |  |  |  |  |  |
Combined lease-related liabilities, non-current portion |  |  |  |  |  | $ 300,000 |  |  |  |
Sublease Agreement Commenced on May 1,2018 [Member] | Restructuring Charges [Member] |  |  |  |  |  |  |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |  |  |  |  |  |  |
Sublease loss |  | $ 600,000 |  |  |  |  |  |  |  |
Berkeley, California [Member] |  |  |  |  |  |  |  |  |  |
Restructuring Cost And Reserve [Line Items] |  |  |  |  |  |  |  |  |  |
Operating leases, number of leased facilities | Facility | 2 |  |  |  |  |  |  |  |  | 2

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