Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (23,879,000)	$ (16,815,000)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	453,000	186,000
Stock-based compensation expense	2,378,000	1,641,000
Amortization of premium (discount) on marketable securities	155,000	196,000
Other non-cash charges	11,000	9,000
Loss on disposal of fixed assets		2,278,000
Premium paid on securities purchased	(264,000)
Changes in operating assets and liabilities:
Accounts receivable		39,000
Prepaid expenses, other current and long-term assets	(517,000)	(572,000)
Accounts payable	138,000	(1,009,000)
Accrued expenses and other current and long-term liabilities	131,000	51,000
Net cash used in operating activities	(21,394,000)	(13,996,000)
Cash flows from investing activities
Purchases of property and equipment	(1,018,000)	(3,094,000)
Change in restricted cash	(120,000)
Purchases of marketable securities	(25,450,000)	(3,006,000)
Sale and redemption of marketable securities	60,328,000	16,097,000
Discount received on securities purchased	29,000
Net cash provided by investing activities	33,769,000	9,997,000
Cash flows from financing activities
Payment of debt issuance costs		(42,000)
Payments on notes payable	(833,000)
Payments of public offering costs	(95,000)
Proceeds from the exercise of stock options	559,000
Net cash used in financing activities	(369,000)	(42,000)
Net increase (decrease) in cash and cash equivalents	12,006,000	(4,041,000)
Cash and cash equivalents at beginning of period	30,915,000	62,944,000
Cash and cash equivalents at end of period	42,921,000	58,903,000
Supplemental disclosures of cash flow information:
Cash paid for interest	469,000	237,000
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	$ 234,000	$ 2,450,000