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Nature of Business (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Debt Instrument        
Cash and cash equivalents $ 58,822 $ 73,488 $ 59,519 $ 73,662
Accumulated deficit $ (218,392) $ (203,689)    
Minimum        
Debt Instrument        
Period over which cash resources will be sufficient to allow Company to fund current operating plan 12 months      
Loan And Security Agreement | Equipment lease facility | Maximum        
Debt Instrument        
Remaining borrowing capacity $ 1,500