Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (70,272)	$ (85,236)	$ (61,706)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,158	1,126	955
Noncash stock-based compensation	9,380	10,573	11,006
Loss on disposal of equipment	49		8
Premium on purchases of marketable securities	(59)	(1,094)	(565)
Amortization of (discount) premium on marketable securities	(641)	1,079	985
Changes in operating assets and liabilities:
Accounts and other receivables	(290)	15,196	(14,750)
Prepaid expenses and other current assets	(242)	(313)	(721)
Accounts payable	(2,940)	(1,577)	2,693
Accrued expenses	1,911	834	(3,707)
Other long-term liabilities	(66)	(66)	149
Net cash used in operating activities	(62,012)	(59,478)	(65,653)
Cash flows from investing activities
Purchase of fixed assets	(515)	(626)	(2,508)
Purchase of marketable securities	(223,060)	(345,490)	(499,115)
Maturities of marketable securities	289,891	372,091	562,292
Net cash provided by investing activities	66,316	25,975	60,669
Cash flows from financing activities
Proceeds from exercise of stock options	2,053	39	108
Proceeds from sale of stock under the employee stock purchase plan	146	225	214
Payment of deferred financing costs		(175)
Borrowings of debt			444
Repayments of debt	(170)	(351)	(246)
Net cash provided by (used in) financing activities	2,029	(262)	520
Net increase (decrease) in cash, cash equivalents and restricted cash	6,333	(33,765)	(4,464)
Cash, cash equivalents and restricted cash, beginning of period	43,648	77,413	81,877
Cash, cash equivalents and restricted cash, end of period	49,981	43,648	77,413
Supplemental disclosure of cash flow information
Cash paid for interest	37	52	46
Supplemental disclosure of noncash investing and financing activities
Cashless exercise of warrants		8,750
Purchases of equipment in accounts payable or accrued expenses	$ 49	$ 36	$ 199