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	$ (73,169)	$ (62,967)	$ (22,811)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	10,605	7,031	2,921
Net amortization of premiums and accretion of discounts on marketable debt securities	1,815	2,004	311
Depreciation and amortization	2,686	544	80
Net realized gains on sales of marketable securities	(479)
Non-cash consideration received for licenses granted	(420)
Other non-cash adjustments	73	(399)	24
Changes in operating assets and liabilities
Accounts receivable	561	1,073	(265)
Prepaid expenses	(3,559)	(755)	(992)
Other current assets	(402)	(159)	(851)
Other assets	(135)	(139)	(168)
Accounts payable	2,621	186	590
Accrued expenses and other current liabilities	1,897	3,873	2,221
Due to related party under services agreement			(34)
Other related party payables			(3,761)
Advance payments		(127)	(26)
Deferred rent	(86)	1,277	246
Net cash used in operating activities	(57,992)	(48,558)	(22,515)
Cash flows from investing activities
Purchases of marketable securities	(68,634)	(45,072)	(163,278)
Maturities of marketable securities	70,224	72,586
Sales of marketable securities	780	23
Purchases of property and equipment	(7,160)	(8,149)	(522)
Restricted cash		(225)
Net cash provided by (used in) investing activities	(4,790)	19,163	(163,800)
Cash flows from financing activities
Proceeds from exercise of stock options	2,493	179	107
Proceeds from issuance of common stock under employee stock purchase plan	556
Proceeds from public offering of common stock, net of underwriting discounts and commissions	81,994
Proceeds from initial public offering of common stock, net of transaction costs			145,184
Issuance costs for public offering of common stock	(445)	(60)
Net cash provided by financing activities	84,598	119	239,310
Net increase (decrease) in cash and cash equivalents	21,816	(29,276)	52,995
Cash and cash equivalents
Beginning of period	24,840	54,116	1,121
End of period	46,656	24,840	54,116
Supplemental cash flow information
Cash paid for interest			7
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses	254	1,181	96
Non-cash consideration received for licenses granted	$ 420
Issuance costs for public offering of common stock in accounts payable and accrued expenses		$ 33
Conversion of related party promissory notes into Series C convertible preferred stock			1,389
Conversion of convertible preferred stock into common stock upon initial public offering			111,392
Series C Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs			26,021
Supplemental disclosures of non-cash investing and financing activities
Conversion of accrued service fees to related party			2,403
Series D Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs			$ 67,998