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	$ (67,543)	$ (31,634)	$ (12,397)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	2,994	1,104	328
Loss on disposal of property and equipment	166	13	9
Equity-based compensation	15,322	6,715	1,308
Changes in operating assets and liabilities:
Accounts receivable	(4,017)	(5,454)	(1,012)
Prepaid expenses and other current assets	(1,893)	(979)	(525)
Accounts payable	(488)	1,784	335
Accrued expenses	2,394	3,050	805
Deferred revenue	(12,988)	67,975	9,312
Other assets	902	(6,435)	(76)
Other long-term liabilities	(125)	(30)	150
Net cash (used in) provided by operating activities	(65,276)	36,109	(1,763)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(10,091)	(6,165)	(2,554)
Net cash used in investing activities	(10,091)	(6,165)	(2,554)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from sale of Class A-1 and A-2 preferred units and Series B preferred stock			74,661
Payments to acquire in-process research and development		(600)	(1,100)
Payment of preferred unit and preferred stock issuance costs		(100)	(2,671)
Proceeds from common stock offering	141,000	170,507
Proceeds from options exercised	1,156	2
Issuance of shares through employee stock purchase plan	825	259
Payment of common stock offering costs		(2,764)	(602)
Net cash provided by financing activities	142,981	167,304	70,288
Net increase in cash and cash equivalents	67,614	197,248	65,971
Cash and cash equivalents, beginning of period	273,064	75,816	9,845
Cash and cash equivalents, end of period	340,678	273,064	75,816
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	$ 805	3,090	219
Acquisition of in-process research and development unpaid at period end			600
Financing costs incurred but unpaid at period end			$ 970
Conversion of convertible preferred stock to common stock		$ 88,557