Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (30,163)	$ (28,645)
Adjustments to reconcile net loss to net cash used in operating activities:
Equity-based compensation expense	2,586	2,925
Depreciation and amortization	62	90
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(624)	(2,744)
Accounts payable	(696)	(3,023)
Accrued expenses	990	4,289
Net cash used in operating activities	(27,845)	(27,108)
Cash flows from investing activities:
Purchases of property and equipment	(19)	(41)
Net cash used in investing activities	(19)	(41)
Cash flows from financing activities:
Proceeds from issuance of common stock under employee stock plans	521	159
Net cash provided by financing activities	521	159
Net decrease in cash and restricted cash	(27,343)	(26,990)
Cash and restricted cash, beginning of period	181,174	139,301
Cash and restricted cash, end of period	153,831	112,311
Supplemental disclosure of cash flow information:
Property and equipment purchases included in accounts payable and accrued expenses	$ 7	47
Initial recognition of right-of-use lease assets		$ 2,215