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Consolidated Statements of Cash Flows - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Operating activities      
Net loss $ (118,183) $ (79,159) $ (56,073)
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation and amortization 1,982 1,073 680
Stock based compensation expense 15,590 11,328 8,668
Loss / (gain) on sale of asset, net   9 (2)
Amortization of debt discount and debt issuance costs 458 390 333
Accrual of the end of term charge on the debt 76 110 160
Changes in operating assets and liabilities:      
Prepaid expenses and other assets (1,484) (2,972) (1,832)
Accounts payable (1,781) 3,312 (1,131)
Accrued expenses and other 2,642 1,493 2,533
Net cash used in operating activities (100,700) (64,416) (46,664)
Investing activities      
Purchase of fixed assets (3,454) (5,351) (826)
Proceeds from sale of asset   10 2
Maturity of a certificate of deposit     2,153
Net cash (used in) / provided by investing activities (3,454) (5,341) 1,329
Financing activities      
Payments on capital lease obligations   (64) (96)
Proceeds from issuance of debt   5,000  
Proceeds from issuance of common stock, net 222,942 109,013 67,017
Proceeds from exercise of stock options 5,112 1,390 1,626
Payment of debt issuance costs (250) (250) (100)
Net cash provided by financing activities 227,804 115,089 68,447
Increase in cash and cash equivalents 123,650 45,332 23,112
Cash and cash equivalents at beginning of period 159,226 113,894 90,782
Cash and cash equivalents at end of period 282,876 159,226 113,894
Supplemental disclosures of cash flow information:      
Cash paid for interest 2,948 1,803 1,809
Cash received for taxes, net $ 3,008 $ 9,429 1,221
Supplemental disclosures of non-cash investing and financing activities:      
Value of warrant exercised by converting the warrant into shares of common stock ("net issuance method")     $ 790