Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Cash flows from operating activities:    
Net loss $ (24,043) $ (19,586)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation and amortization 1,314 1,041
Stock-based compensation 2,045 5,195
Change in fair value of warrant liability   273
Other non-cash operating activities, net 55 93
Increase (decrease) in cash and cash equivalents from:    
Accounts receivable (1,172) (1,384)
Prepaid expenses and other current assets 731 (870)
Accounts payable and accrued expenses 767 737
Deferred revenue (1,665) 7,305
Net cash used in operating activities (21,968) (7,196)
Cash flows from investing activities:    
Purchases of property and equipment (2,588) (1,826)
Proceeds from sales of short-term investments 11,759  
Proceeds from maturities of short-term investments 18,455  
Purchases of short-term investments (31,889)  
Proceeds from repayment of stockholder loans   712
Other investing activities, net (184) 77
Net cash used in investing activities (4,447) (1,037)
Cash flows from financing activities:    
(Payments) proceeds of initial public offering, net (265) 63,942
Proceeds from issuance of common stock 234 228
Payments under capital lease (38) (38)
Borrowings under credit facility   4,500
Repayments of credit facility (806) (3,326)
Net cash (used in) provided by financing activities (875) 84,088
Effect of exchange rate on cash (562) (26)
(Decrease) increase in cash and cash equivalents (27,852) 75,829
Cash and cash equivalents-Beginning of year 51,612 4,886
Cash and cash equivalents-End of period 23,760 80,715
Noncash investing activities:    
Property and equipment purchases included in accounts payable 21 160
Noncash financing activities:    
Accretion of redeemable convertible preferred stock   3,704
Conversion of redeemable convertible preferred stock to common stock   83,515
Initial public offering costs incurred but unpaid at period end   1,418
Supplementary cash flow information-Interest paid 486 309
Series D Redeemable Convertible Preferred Stock [Member]
   
Cash flows from financing activities:    
Proceeds from issuance of Series D and BRN redeemable convertible preferred stock-net   17,282
Series BRN Preferred Stock [Member]
   
Cash flows from financing activities:    
Proceeds from issuance of Series D and BRN redeemable convertible preferred stock-net   $ 1,500