Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net loss	$ (10,458)	$ (17,782)
Adjustments to reconcile net loss to net cash used in operating activities
Non-cash interest expense	121
Depreciation and amortization expense	1,219	1,239
Gain on sale of property and equipment	(574)
Premiums paid on marketable securities		(51)
Amortization and accretion on marketable securities	(216)	(141)
Stock-based compensation expense	594	764
Loss on equity method investment	301	746
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(183)	801
Accounts payable, accrued expenses and other	(4,456)	(2,397)
Deferred rent		(518)
Net cash used in operating activities	(13,652)	(17,339)
Cash flows from investing activities
Purchase of property and equipment		(16)
Proceeds on sale of property and equipment	615
Proceeds from maturities and sales of marketable securities	29,500
Purchases of marketable securities		(41,818)
Net cash provided by (used in) investing activities	30,115	(41,834)
Net increase (decrease) in cash, cash equivalents and restricted cash	16,463	(59,173)
Cash, cash equivalents and restricted cash, beginning of period	20,663	94,217
Cash, cash equivalents and restricted cash, end of period	37,126	35,044
Non-cash investing activities
Purchases of property and equipment in accounts payable, accrued expenses and other		$ 114
Supplemental disclosure of cash flows
Cash paid for income taxes	10
Cash paid for interest	$ 361