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Income Taxes (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Taxes [Line Items]        
Increase in valuation allowance $ 317,400 $ 285,600    
Benefit of federal net operating loss deferred tax assets 6,000      
Benefit of state net operating loss deferred tax assets 400      
Total unrecognized tax benefit $ 16,785 18,735 $ 16,952 $ 11,634
Period of increase or decrease in total amount of unrecognized income tax benefits 12 months      
Accrued interest or penalties $ 0 $ 0    
Domestic Tax Authority [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards $ 684,000      
Net operating loss carryforward, expiration 2024      
Research and development credit carryforward $ 26,900      
Research and developmen tax credit carryforward, expiration 2024      
State and Local Jurisdiction [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards $ 574,100      
Net operating loss carryforward, expiration 2016      
Research and development credit carryforward $ 29,000