Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (12,824)	$ (14,346)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,394	1,278
Depreciation and amortization expense	47	8
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3)	3
Accounts payable	(1,341)	523
Accrued expense	(3,277)	1,113
Net cash used in operating activities	(14,004)	(11,421)
Cash flows from investing activities:
Purchases of marketable securities	(251,869)	(31,113)
Sales and maturities of marketable securities	68,834	20,033
Purchases of property and equipment, net of disposals	(1)	(125)
Net cash used in investing activities	(183,036)	(11,205)
Cash flows from financing activities:
Proceeds from issuance of stock, net of transaction costs	311,825	38,278
Proceeds from exercise of common stock options	1,449
Net cash provided by financing activities	313,274	38,278
Net increase in cash and cash equivalents	116,234	15,652
Cash and cash equivalents at beginning of period	148,627	19,145
Cash and cash equivalents at end of period	$ 264,861	$ 34,797