Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (19,881)	$ (24,333)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,959	2,065
Depreciation and amortization expense	1,802	2,006
Non-cash operating lease cost	536	535
Accretion of discount on investments	(66)
Non-cash interest expense	314
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	203	(643)
Accounts receivable	(237)	(6,667)
Deferred revenue	(6,634)	(208)
Accounts payable	590	(2,474)
Operating lease liabilities	(1,066)	(1,024)
Accrued expenses and other current and long-term liabilities	(1,771)	(1,650)
Net cash (used in) operating activities	(24,251)	(32,393)
Cash flows from investing activities:
Purchases of property and equipment	(71)	(306)
Purchases of investments	(12,931)
Sales and maturities of investments	22,439
Net cash (used in) provided by investing activities	9,437	(306)
Cash flows from financing activities:
Proceeds from exercise of stock options	60	120
Proceeds from issuance of common stock and restricted common stock	120	153
Proceeds from at the market equity offering, net of commissions	4,116
Issuance of common stock under ESPP plan	249	207
Net cash provided by financing activities	4,545	480
Net increase (decrease) in cash and cash equivalents	(10,269)	(32,219)
Cash, cash equivalents and restricted cash at beginning of period	65,126	85,933
Cash, cash equivalents and restricted cash at end of period	54,857	53,714
Supplemental disclosure of cash flow information:
Cash paid for interest	610
Supplemental disclosure of non-cash investing and financing activities:
Unsettled issuance of common stock from the at the market offering	62
Property and equipment purchases included in accounts payable and accrued expenses	$ 133	$ 134