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	$ (137,481)	$ (71,894)	$ (46,315)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	2,008	1,151	238
Stock-based compensation expense	11,542	6,770	4,583
Other non-cash charges		37	40
Amortization of premium (discount) on marketable	333	729	871
Loss on disposal of fixed assets		2,283	150
Amortization of convertible debt discount and debt issuance costs	4,826
Premium paid on securities purchased	(857)	(543)
Changes in operating assets and liabilities:
Accounts receivable	(410)	95
Inventory	(1,799)
Prepaid expenses, other current and long-term assets	387	(2,873)	(526)
Accounts payable	4,188	(993)	1,581
Accrued expenses and other current and long-term liabilities	9,432	2,766	429
Net cash used in operating activities	(107,831)	(62,472)	(38,949)
Cash flows from investing activities
Purchases of property and equipment	(2,146)	(8,440)	(5,197)
Change in restricted cash	(120)	(400)	48
Purchases of marketable securities	(356,754)	(196,061)	(145,798)
Sale and redemption of marketable securities	240,228	72,147	137,702
Net cash used in investing activities	(118,792)	(132,754)	(13,245)
Cash flows from financing activities
Proceeds from the issuance of 2024 convertible notes	201,250
Payment of debt issuance costs	(6,470)	(42)	(108)
Proceeds from the offering of common stock	132,666	147,889
Payments on notes payable	(8,333)		(3,500)
Proceeds from issuance of notes payable		15,000	15,004
Payments of public offering costs	(490)	(293)	(225)
Proceeds from the exercise of stock options	3,858	167	593
Proceeds from Employee Stock Purchase Plan	1,016	476	276
Net cash provided by financing activities	323,497	163,197	12,040
Net increase (decrease) in cash and cash equivalents	96,874	(32,029)	(40,154)
Cash and cash equivalents at beginning of period	30,915	62,944	103,098
Cash and cash equivalents at end of period	127,789	30,915	62,944
Supplemental disclosures of cash flow information:
Cash paid for interest	5,080	1,297	572
Supplemental disclosures of non-cash financing activities:
Public offering costs included in accounts payable or accrued		95
Portion of debt proceeds allocated to equity component	62,466
Purchases of property and equipment in accounts payable and accrued expenses	$ 9	$ 622	$ 1,576