CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (65,281)	$ (71,770)
Adjustments to reconcile net loss to cash used in operating activities:
Stock-based compensation expense	8,503	5,916
Depreciation and amortization	340	452
Non-cash rent expense	(114)	253
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(352)	(4,048)
Accounts payable, accrued expenses and other current liabilities	(8,923)	12,273
Net cash used in operating activities	(65,827)	(56,924)
Investing activities
Purchases of short-term investments	(43,413)	(69,875)
Maturities of short-term investments	105,147	142,687
Purchases of property and equipment		(3,126)
Net cash provided by investing activities	61,734	69,686
Financing activities
Proceeds from the exercise of stock options	566	519
Proceeds from issuance of common stock from ESPP	324	295
Net cash provided by financing activities	890	814
Net (decrease) increase in cash, cash equivalents and restricted cash	(3,203)	13,576
Cash, cash equivalents and restricted cash at beginning of year	62,697	49,943
Cash, cash equivalents and restricted cash at end of year	$ 59,494	$ 63,519