Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net loss	$ (52,764,000)	$ (31,000,000)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	927,000	269,000
Stock-based compensation expense	4,741,000	3,260,000
Amortization of premium (discount) on marketable securities	259,000	345,000
Loss on disposal of fixed assets		2,278,000
Amortization of convertible debt discount and debt issuance costs	1,190,000	18,000
Premium paid on securities purchased	(580,000)	(22,000)
Changes in operating assets and liabilities:
Accounts receivable		(6,000)
Prepaid expenses, other current and long-term assets	1,456,000	(353,000)
Accounts payable	736,000	412,000
Accrued expenses and other current and long-term liabilities	1,796,000	(517,000)
Net cash used in operating activities	(42,239,000)	(25,316,000)
Cash flows from investing activities
Purchases of property and equipment	(1,682,000)	(7,678,000)
Change in restricted cash	(120,000)
Purchases of marketable securities	(118,320,000)	(10,804,000)
Sale and redemption of marketable securities	135,363,000	21,997,000
Net cash provided by investing activities	15,241,000	3,515,000
Cash flows from financing activities
Proceeds from the issuance of 2024 convertible notes	201,250,000
Payment of debt issuance costs	(6,470,000)	(42,000)
Proceeds from the offering of common stock		77,644,000
Payments on notes payable	(3,333,000)
Payments of public offering costs	(95,000)	(31,000)
Proceeds from the exercise of stock options	1,457,000	56,000
Proceeds from Employee Stock Purchase Plan	453,000	240,000
Net cash provided by financing activities	193,262,000	77,867,000
Net increase in cash and cash equivalents	166,264,000	56,066,000
Cash and cash equivalents at beginning of period	30,915,000	62,944,000
Cash and cash equivalents at end of period	197,179,000	119,010,000
Supplemental disclosures of cash flow information:
Cash paid for interest	921,000	477,000
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	$ 66,000	$ 154,000