CONSOLIDATED STATEMENT OF CASH FLOWS - USD ($)	10 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (8,010,860)	$ (39,493,123)	$ (29,141,630)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	11,997	277,231	49,881
Stock-based compensation expense	1,462,061	6,572,963	6,597,359
Amortization of premium on investments	0	16,161	9,523
Other non-cash items	55,221	3,434	4,123
Changes in operating assets and liabilities:
Restricted cash	(126,808)	240	(27)
Accounts receivable	0	(12,181)	0
Inventory	0	(454,132)	0
Prepaid expenses and other current assets	(370,396)	(17,409)	(538,178)
Other assets	(50,000)	(64,800)	100,103
Accounts payable	254,253	309,437	621,393
Accrued expenses and other current liabilities	220,375	746,879	1,570,216
Deferred revenue	0	88,344	0
Deferred rent	57,849	(9,475)	(18,881)
Other long term liabilities	15,442	(15,442)	0
Net cash used in operating activities	(6,480,866)	(32,051,873)	(20,746,118)
Investing activities
Purchases of marketable securities	0	(38,682,081)	(39,397,769)
Proceeds from maturities and sales of marketable securities	0	26,995,324	12,398,295
Purchases of property and equipment	(76,141)	(559,378)	(265,617)
Proceeds from sales of property and equipment	0	5,255	0
Net cash used in investing activities	(76,141)	(12,240,880)	(27,265,091)
Financing activities
Proceeds from initial public offering, net of offering costs	0	0	80,435,430
Proceeds from sale of restricted common stock to founders	2,321	0	0
Proceeds from exercise of common stock		22,098	8,321
Proceeds from early exercise of common stock	8,138	0	400,000
Deferred IPO issuance costs	(575,245)	0	0
Net cash provided by financing activities	40,411,160	22,098	80,843,751
Net (decrease) increase in cash and cash equivalents	33,854,153	(44,270,655)	32,832,542
Cash and cash equivalents at beginning of period		66,686,695	33,854,153
Cash and cash equivalents at end of period	33,854,153	22,416,040	66,686,695
Supplemental cash flow information
Property and equipment purchases included in accounts payable and accrued expense	21,000	7,100	106,680
IPO issuance costs paid in cash through December 31, 2014	0	0	575,245
IPO issuance costs included in accounts payable and accrued expenses	499,549	0	0
Issuance of Series A convertible preferred stock in satisfaction of accounts payable	55,221	0	0
Series A Preferred Stock
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs	15,581,811	0	0
Series B Preferred Stock
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs	$ 25,394,135	$ 0	$ 0