CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities
Net loss	$ (34,164)	$ (28,974)
Adjustments to reconcile net loss to cash used in operating activities:
Stock-based compensation expense	5,475	2,644
Depreciation and amortization	167	245
Non-cash rent expense	(56)	102
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(847)	(3,321)
Accounts payable, accrued expenses and other current liabilities	(6,673)	(2,074)
Net cash used in operating activities	(36,098)	(31,378)
Investing activities
Purchases of short-term investments	(15,370)	(55,819)
Maturities of short-term investments	44,243	80,171
Purchases of property and equipment		(811)
Net cash used in investing activities	28,873	23,541
Financing activities
Proceeds from the exercise of stock options	383	21
Proceeds from issuance of common stock from ESPP	324	295
Net cash provided by financing activities	707	316
Net increase in cash, cash equivalents and restricted cash	(6,518)	(7,521)
Cash, cash equivalents and restricted cash at beginning of year	62,697	49,943
Cash, cash equivalents and restricted cash at end of year	$ 56,179	$ 42,422