Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (62,967,000)	$ (22,811,000)	$ (4,003,000)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	7,031,000	2,921,000	319,000
Net amortization of premiums and accretion of discounts on marketable debt securities	2,004,000	311,000
Depreciation and amortization	544,000	80,000	0
Other non-cash adjustments	(399,000)	24,000	175,000
Changes in operating assets and liabilities
Accounts receivable	1,073,000	(265,000)	(838,000)
Prepaid expenses	(755,000)	(992,000)	(28,000)
Other current assets	(159,000)	(851,000)
Other assets	(139,000)	(168,000)
Accounts payable	186,000	590,000	33,000
Accrued expenses and other current liabilities	3,873,000	2,221,000	764,000
Due to related party under services agreement		(34,000)	768,000
Other related party payables		(3,761,000)	258,000
Advance payments	(127,000)	(26,000)	153,000
Deferred rent	1,277,000	246,000
Net cash used in operating activities	(48,558,000)	(22,515,000)	(2,399,000)
Cash flows from investing activities
Restricted cash	(225,000)
Purchases of marketable securities	(45,072,000)	(163,278,000)
Maturities of marketable securities	72,586,000
Sales of marketable securities	23,000
Purchases of property and equipment	(8,149,000)	(522,000)
Net cash provided by (used in) investing activities	19,163,000	(163,800,000)
Cash flows from financing activities
Proceeds from exercise of stock options	179,000	107,000	1,000
Issuance costs for potential public offering of common stock	(60,000)
Proceeds from initial public offering of common stock, net of transaction costs		145,184,000
Proceeds from related party promissory notes			2,400,000
Net cash provided by financing activities	119,000	239,310,000	2,401,000
Net increase (decrease) in cash and cash equivalents	(29,276,000)	52,995,000	2,000
Cash and cash equivalents
Beginning of period	54,116,000	1,121,000	1,119,000
End of period	24,840,000	54,116,000	1,121,000
Supplemental cash flow information
Cash paid for interest		7,000	164,000
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses	1,181,000	96,000
Deferred issuance costs for potential public offering of common stock in accrued expenses	$ 33,000
Conversion of related party promissory notes into Series C convertible preferred stock		1,389,000
Conversion of convertible preferred stock into common stock upon initial public offering		111,392,000
Deferred issuance costs for Series C convertible preferred stock in accrued expenses			$ 157,000
Series C Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs		26,021,000
Supplemental disclosures of non-cash investing and financing activities
Conversion of accrued service fees to related party		2,403,000
Series D Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs		$ 67,998,000