Commitments and Contingencies - Additional Information (Details) ft² in Thousands, $ in Thousands	3 Months Ended	12 Months Ended		24 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) officer	May 31, 2018 ft²	Aug. 31, 2015 ft²
Commitments And Contingencies [Line Items]
Property and equipment, net		$ 15,868	$ 16,624	$ 15,868
Facility financing obligation		7,998	7,998	$ 7,998
Interest expense		1,047	1,048
Rent expense for operating leases		308	440
Three Former Officers
Commitments And Contingencies [Line Items]
Number of former officers | officer				3
Severance expenses		332	793
Stock compensation expense related to accelerated vesting of former officers stock options		212	374
November 2018 Employee Restructuring
Commitments And Contingencies [Line Items]
Total employee severance costs		306
Severance costs expensed during the period		196
Accrued severance costs related to November 2018 realignment		37		$ 37
June 2017 Employee Restructuring
Commitments And Contingencies [Line Items]
Severance costs expensed during the period	$ 224
Primary Facility Lease
Commitments And Contingencies [Line Items]
Rentable square feet of facility space | ft²						51,000
Optional term of extending lease agreement						5 years
Current contractual base rent payments per month		$ 95
Percentage of increase in annual rental payments		3.00%
Grounds rent expense per month		$ 8
Facility financing obligation		7,998	7,998	7,998
Interest expense		1,044	1,044
Rent expense for operating leases		308	355
Primary Facility Lease | Other Accrued Expenses
Commitments And Contingencies [Line Items]
Accrued interest		$ 41	37	41
Primary Facility Lease | Building Related to Facility Lease Obligation
Commitments And Contingencies [Line Items]
Property and equipment, estimated useful life		25 years
Property and equipment, net		$ 10,557	$ 10,557	10,557
Primary Facility Lease | Sublease Agreement
Commitments And Contingencies [Line Items]
Rentable square feet of facility space | ft²					6,400
Percentage of increase in annual rental payments		3.00%
Annual sublease rental income, subject to termination of primary facility lease		$ 141		$ 141
Primary Facility Lease | Sublease Agreement | Other income and expense
Commitments And Contingencies [Line Items]
Sublease rental income		$ 59