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	$ (37,545)	$ (25,567)	$ (9,518)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	645	336	94
Amortization of intangible assets	665	665	665
Loss on disposal of fixed assets	28
Stock-based compensation	12,338	10,926	870
Net premium amortization on investments	(140)	26	4
Landlord incentive for tenant improvements		2,119
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(367)	(144)	(415)
Accounts payable	490	517	3
Accrued expenses	387	245	246
Other asset		(101)
Deferred rent	(397)	(109)
Net cash used in operating activities	(23,896)	(11,087)	(8,051)
Cash flows from investing activities:
Purchases of property and equipment	(276)	(2,551)	(64)
Purchases of available-for-sale securities	(40,297)	(43,595)	(19,067)
Maturities of available-for-sale securities	41,815	23,148	4,750
Sales of investments	24,875
Net cash provided by investing activities	26,117	(22,998)	(14,381)
Cash flows from financing activities:
Proceeds from exercises of stock options and warrants	498	539
Proceeds from issuance of common stock from private offering, net of issuance costs		34,843
Proceeds from issuance of common stock from initial public offering, net of issuance costs			20,915
Proceeds from issuance of common stock under employee stock plan	327
Proceeds from issuance of common stock from rights offering, net of issuance costs	44,786
Tax payments related to shares withheld for vested restricted stock units	(115)
Net cash provided by financing activities	45,496	35,382	20,915
Net increase in cash and cash equivalents	47,717	1,297	(1,517)
Cash and cash equivalents at beginning of period	3,386	2,089	3,606
Cash and cash equivalents at end of period	51,103	3,386	2,089
Supplemental disclosure of noncash investing and financing activities:
Reclassification of deferred offering costs to additional paid-in capital upon initial public offering			627
Equipment purchased in accounts payable and accrued expenses	$ 33	$ 38	$ 18