Significant Accounting Policies (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents
Cash and cash equivalents	$ 120,738,000				$ 158,708,000				$ 120,738,000	$ 158,708,000	$ 131,490,000	$ 195,620,000
Leasehold Improvements and Equipment
Deferred rents	998,000				1,100,000				998,000	1,100,000
Revenue Recognition
Grant and licensing revenue	202,000	$ 120,000	$ 2,252,000	$ 9,900,000	10,098,000	$ 5,482,000	$ 10,370,000	$ 2,761,000	12,474,000	28,711,000	$ 35,768,000
Receivables from government contracts	309,000				9,300,000				309,000	9,300,000
Unbilled expenses	0				465,000				0	465,000
Effective Income Tax Rate Reconciliation, percent
Uncertain Tax Positions	653,000				653,000				$ 653,000	653,000
Stock Option Valuation
Award vesting percentage									25.00%
Award vesting period									36 months
Expected dividend yield									0.00%
Notes payable and capital lease obligations	104,000				271,000				$ 104,000	$ 271,000
Future minimum lease payments	11,417,000								$ 11,417,000
Customer Concentration Risk | Sales Revenue, Net | Genentech
Stock Option Valuation
Concentration risk percentage									0.40%	1.20%	7.60%
Customer Concentration Risk | Sales Revenue, Net | Merck
Stock Option Valuation
Concentration risk percentage									9.20%	0.20%	2.30%
Computer and office equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life									3 years
Computer and office equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful life									5 years
Lab and leased equipment
Property, Plant and Equipment [Line Items]
Useful life									5 years
Contract manufacturing organization equipment
Property, Plant and Equipment [Line Items]
Useful life									5 years
Accrued Expenses
Revenue Recognition
Accrued expenses for subcontractor fees incurred under government contracts	54,000				1,800,000				$ 54,000	$ 1,800,000
Accounts Payable
Revenue Recognition
Subcontractor fees in accounts payable	$ 161,000				$ 4,900,000				161,000	4,900,000
Grant [Member]
Revenue Recognition
Grant and licensing revenue									$ 11,268,000	$ 28,321,000	$ 32,242,000