Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (41,569)	$ (23,879)	$ (137,481)	$ (71,894)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	563	453
Stock-based compensation expense	3,657	2,378
Other non-cash charges		11
Amortization of (discount) premium on marketable securities	(230)	155
Amortization of debt discount and debt issuance costs	1,884
Premium paid on securities purchased	(3)	(264)
Changes in operating assets and liabilities:
Accounts receivable	(1,987)
Inventory	(726)
Prepaid expenses, other current and long-term assets	(620)	(517)
Accounts payable	(2,806)	138
Accrued expenses and other current and long-term liabilities	(3,144)	131
Net cash used in operating activities	(44,981)	(21,394)
Cash flows from investing activities
Purchases of property and equipment	(334)	(1,018)
Purchases of marketable securities	(32,546)	(25,421)
Sale and redemption of marketable securities	99,462	60,328
Net cash provided by investing activities	66,582	33,889
Cash flows from financing activities
Payments on notes payable	(2,500)	(833)
Payments of public offering costs		(95)
Proceeds from the exercise of stock options	414	559
Net cash used in financing activities	(2,086)	(369)
Net increase in cash, cash equivalents, and restricted cash	19,515	12,126
Cash, cash equivalents, and restricted cash at beginning of period	128,389	31,395	31,395
Cash, cash equivalents, and restricted cash at end of period	147,904	43,521	$ 128,389	$ 31,395
Supplemental disclosures of cash flow information:
Cash paid for interest	326	469
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	$ 13	$ 234