Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (85,343)	$ (67,543)	$ (31,634)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	4,464	2,994	1,104
Loss on disposal of property and equipment	75	166	13
Equity-based compensation	17,046	15,322	6,715
Accretion of investment discounts	(676)
Changes in operating assets and liabilities:
Accounts receivable	2,924	(4,017)	(5,454)
Prepaid expenses and other current assets	310	(1,893)	(979)
Accounts payable	232	(488)	1,784
Accrued expenses	2,780	2,394	3,050
Deferred revenue	(3,936)	(12,988)	67,975
Other assets	1,022	902	(6,435)
Other long-term liabilities	(155)	(125)	(30)
Net cash (used in) provided by operating activities	(61,257)	(65,276)	36,109
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(6,358)	(10,091)	(6,165)
Proceeds from sale of property and equipment	131
Purchases of marketable securities	(254,555)
Net cash used in investing activities	(260,782)	(10,091)	(6,165)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payments to acquire in-process research and development			(600)
Payment of preferred unit and preferred stock issuance costs			(100)
Proceeds from common stock offering	28,971	141,000	170,507
Proceeds from options exercised	10,652	1,156	2
Issuance of shares through employee stock purchase plan	1,018	825	259
Payment of common stock offering costs	(424)		(2,764)
Net cash provided by financing activities	40,217	142,981	167,304
Net (decrease) increase in cash and cash equivalents	(281,822)	67,614	197,248
Cash and cash equivalents, beginning of period	340,678	273,064	75,816
Cash and cash equivalents, end of period	58,856	340,678	273,064
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	$ 1,071	$ 805	3,090
Conversion of convertible preferred stock to common stock			$ 88,557