Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (57,968)	$ (53,671)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	868	858
Non-cash stock-based compensation	4,861	7,496
Premium on purchases of marketable securities	(378)	(59)
Amortization of (discount) premium on marketable securities	(1,875)	(166)
Loss on disposal of fixed assets	381
Impairment of fixed assets	128
Changes in operating assets and liabilities:
Accounts and other receivables	114	(22)
Prepaid expenses and other assets	464	(477)
Accounts payable	815	(3,630)
Operating lease right-of-use asset	790
Operating lease liability	(783)
Accrued expenses	4,327	(590)
Other liabilities	(17)	(49)
Net cash used in operating activities	(48,273)	(50,310)
Cash flows from investing activities
Purchases of fixed assets	(177)	(468)
Purchases of marketable securities	(188,727)	(173,029)
Proceeds from sale of fixed assets	2
Maturities of marketable securities	227,155	214,749
Net cash provided by investing activities	38,253	41,252
Cash flows from financing activities
Proceeds from exercise of stock options	3,810	1,883
Proceeds from sale of common stock under Employee Stock Purchase Plan	79	102
Repayments of debt	(131)	(132)
Net cash provided by financing activities	3,758	1,853
Net decrease in cash and cash equivalents	(6,262)	(7,205)
Cash, cash equivalents and restricted cash, beginning of period	49,981	43,648
Cash, cash equivalents and restricted cash, end of period	43,719	36,443
Supplemental disclosure of cash flow information
Cash paid for interest	18	26
Supplemental disclosure of non-cash activities
Purchases of fixed assets in accounts payable and accrued expenses	11	$ 11
Operating lease right-of-use asset and liability (non-cash adoption balances)	1,889
Noncash change in right of use asset and liability from remeasurement of existing leases and addition of new lease	$ 1,166