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Income Taxes - Additional Information (Details) - USD ($) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 01, 2017
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Income tax expense | $ (44,000) | $ 1,706,000 |  | $ 1,662,000 | $ 0 | $ 0 |
Federal statutory income tax rate |  |  |  | 34.00% | 34.00% | 34.00% |
(Decrease) increase in the valuation allowance |  |  |  | $ (166,100,000) | $ 8,200,000 | $ 19,600,000 |
Tax cuts and jobs act of 2017 change in tax rate income tax expense benefit |  |  |  | 72,700,000 |  |  |
Net operating loss carryforwards | 4,701,000 |  |  | 4,701,000 | 123,672,000 |  |
Exclude net operating tax credit carryforwards | 12,225,000 |  |  | 12,225,000 | 25,297,000 |  |
Unrecognized tax benefits that would impact effective tax rate | 4,500,000 |  |  | 4,500,000 |  |  |
Accrued interest or penalties related to uncertain tax positions | 0 |  |  | 0 |  |  |
Federal Orphan Credit [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Tax credit carry forward amount | 1,000,000 |  |  | $ 1,000,000 |  |  |
Tax credit carryforward expiration year |  |  |  | 2037 |  |  |
State [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Net operating loss carry-forwards | 27,300,000 |  |  | $ 27,300,000 |  |  |
Net operating loss carryforwards expiration year |  |  |  | 2028 |  |  |
California [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Net operating loss carry-forward expired |  |  |  | $ 24,300,000 | $ 41,200,000 | $ 22,400,000 |
California [Member] | Research and Development [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Tax credit carry forward amount | 19,800,000 |  |  | 19,800,000 |  |  |
Federal [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Net operating loss carryforwards | 119,800,000 |  |  | 119,800,000 |  |  |
Exclude net operating tax credit carryforwards | 15,400,000 |  |  | 15,400,000 |  |  |
Net operating loss carry-forwards | $ 13,600,000 |  |  | $ 13,600,000 |  |  |
Net operating loss carryforwards expiration year |  |  |  | 2036 |  |  |
ASU 2016-09 [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Net operating loss carryforwards |  |  |  |  |  |  | $ 1,800,000
Operating loss carryforwards, valuation allowance |  |  |  |  |  |  | $ 1,800,000
Scenario, Plan [Member] |  |  |  |  |  |  |
Operating Loss Carryforwards [Line Items] |  |  |  |  |  |  |
Federal statutory income tax rate |  |  | 21.00% |  |  |  |
Tax cuts and jobs act of 2017 change in tax rate income tax expense benefit |  |  | $ 51,800,000 |  |  |  |

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