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Consolidated Statements of Cash Flows - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash flows from operating activities:      
Net loss $ (50,975) $ (82,260) $ (73,219)
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation and amortization 1,650 2,238 1,286
Impairment of property and equipment 0 147 0
Impairment of property and equipment related to restructuring 422 1,994 0
Amortization of premium on debt securities 59 659 1,116
Stock-based compensation expense 6,893 9,215 19,337
Issuance of common stock for board of directors fees 129 154 165
Net loss on equity method investment 1,018 1,542 1,561
Changes in operating assets and liabilities:      
Prepaid expenses and other assets 937 946 (1,113)
Accounts payable 68 (1,178) (171)
Accrued expenses, current and other non-current liabilities (5,752) 4,809 752
Net cash used in operating activities (45,551) (61,734) (50,286)
Cash flows from investing activities:      
Investment in joint venture (1,100) (1,750) (1,500)
Purchases of property and equipment (158) (2,586) (5,229)
Maturities of short-term investments 87,789 72,013 53,528
Sales of short-term investments 0 23,089 10,817
Purchases of short-term investments (68,857) (82,671) (95,225)
(Decrease) increase in restricted cash (350) 197 (681)
Net cash provided by investing activities 17,324 8,292 (38,290)
Cash flows from financing activities:      
Net proceeds from the issuance of common stock 0 53,925 124,063
Issuances of common stock under benefit plans, net of withholding taxes paid 0 223 1,323
Net cash provided by financing activities 0 54,148 125,386
Net increase (decrease) in cash, cash equivalents and restricted cash (28,227) 706 36,810
Cash and cash equivalents at beginning of period 43,930 43,224 6,414
Cash and cash equivalents at end of period 15,703 43,930 43,224
Supplemental disclosure of non-cash investing activity      
Additions of property and equipment included in accounts payable and accrued liabilities $ 25 $ 55 $ 1,003