Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (15,080)	$ (6,364)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,022	1,167
Depreciation and amortization expense	28	23
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	888	290
Accounts payable	1,889	(509)
Accrued expense	(179)	(3,251)
Accrued interest, net of interest received on maturity of investments	(46)
Net cash used in operating activities	(6,478)	(8,644)
Cash flows from investing activities:
Purchases of marketable securities	(173,837)	(122,193)
Sales and maturities of marketable securities	170,387	20,071
Purchases of property and equipment, net of disposals	(41)	(1)
Net cash used in investing activities	(3,491)	(102,123)
Cash flows from financing activities:
Proceeds from exercise of common stock options	83	265
Net cash provided by financing activities	83	265
Net decrease in cash and cash equivalents	(9,886)	(110,502)
Cash and cash equivalents at beginning of period	57,379	148,627
Cash and cash equivalents at end of period	47,493	$ 38,125
Supplemental disclosure of cash flow information:
Acquisition of a right-of-use asset in exchange for a lease liability	$ 900