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Commitments and Contingencies (Narrative) (Details)
ft² in Thousands
 3 Months Ended 6 Months Ended
Jul. 22, 2015
USD ($)
Jun. 30, 2020
USD ($)
ft²
Jun. 30, 2019
USD ($)
Jun. 30, 2020
USD ($)
ft²
Jun. 30, 2019
USD ($)
May 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
May 01, 2019
USD ($)
Mar. 31, 2019
USD ($)
Oct. 31, 2015
USD ($)
Commitments and Contingencies [Line Items]                    
Long-term restricted cash   $ 3,500,000   $ 3,500,000   $ 3,500,000 $ 4,000,000 $ 4,000,000.0 $ 4,500,000 $ 4,500,000
Rent payments   1,800,000   3,600,000            
Operating lease cost   1,600,000 $ 1,600,000 3,100,000 $ 3,100,000          
Additional liability associated with indemnification obligations   $ 0   $ 0            
1305 O’Brien Drive, Menlo Park, California [Member]                    
Commitments and Contingencies [Line Items]                    
Net rentable area | ft²   180   180            
Renewal term   5 years   5 years            
Weighted average remaining lease term   7 years 3 months 18 days   7 years 3 months 18 days            
Weighted average discount rate   7.90%   7.90%            
O’Brien Lease Agreement [Member]                    
Commitments and Contingencies [Line Items]                    
Lease term 132 months                  
Rent expense first twelve months $ 540,000                  
Rent expense last twelve months $ 711,000                  
Period in which payment must be received without incurring late charge 5 days                  
Late charge, percent 5.00%                  
Amount not paid after written notice, bear interest from date due until date paid, period 10 days                  
O’Brien Lease Agreement [Member] | Prime Rate [Member]                    
Commitments and Contingencies [Line Items]                    
Basis spread on variable rate 2.00%