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Income Taxes (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income Taxes [Line Items]        
Income before taxes from U.S. operations $ 28,900 $ (84,800) $ (103,100)  
Income before taxes from foreign operations $ 600 $ 900 $ 800  
Statutory tax rate 21.00% 21.00% 21.00%  
Valuation allowance $ 300,505 $ 298,658    
Net operating loss carryforwards 233,200      
Research and development credit carryforward 49,200      
Total unrecognized tax benefit 5,954 21,979 $ 20,447 $ 18,786
Decrease in balance related to tax positions taken in prior year 17,255  
Accrued interest or penalties 0 $ 0    
Federal [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards 913,900      
Research and development credit carryforward $ 30,700      
Research and developmen tax credit carryforward, expiration 2024      
Federal [Member] | With Expiration [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards $ 755,900      
Federal [Member] | No Expiration [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards 158,000      
State [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards $ 634,300      
Net operating loss carryforward, expiration 2025      
Research and development credit carryforward $ 30,100      
Minimum [Member] | Federal [Member]        
Income Taxes [Line Items]        
Net operating loss carryforward, expiration 2024      
Maximum [Member] | Federal [Member]        
Income Taxes [Line Items]        
Net operating loss carryforward, expiration 2037