Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (40,595)	$ (35,092)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,873	4,162
Depreciation and amortization expense	3,493	3,927
Non-cash operating lease cost	1,100	1,097
Accretion of discount on investments	(102)
Non-cash interest expense	215
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,928)	1,685
Accounts receivable	(1,144)	(1,604)
Deferred revenue	(11,837)	(11,628)
Accounts payable	(700)	(1,645)
Operating lease liabilities	(2,163)	(2,079)
Accrued expenses and other current and long-term liabilities	(29)	(3,004)
Net cash (used in) operating activities	(49,817)	(44,181)
Cash flows from investing activities:
Purchases of property and equipment	(299)	(647)
Purchases of investments	(12,931)
Sales and maturities of investments	40,318
Net cash provided by (used in) provided by investing activities	27,088	(647)
Cash flows from financing activities:
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs		60,759
Proceeds from exercise of stock options	75	120
Proceeds from issuance of common stock and restricted common stock	120	153
Proceeds from at the market equity offering, net of commissions	18,695
Issuance of common stock under ESPP plan	249	207
Net cash provided by financing activities	19,139	61,239
Net (decrease) increase in cash and cash equivalents	(3,590)	16,411
Cash, cash equivalents and restricted cash at beginning of period	65,126	85,933
Cash, cash equivalents and restricted cash at end of period	61,536	102,344
Supplemental disclosure of cash flow information:
Cash paid for interest	1,226
Supplemental disclosure of non-cash investing and financing activities:
Unsettled issuance of common stock from the at the market offering	331
Issuance costs from public offering included in accounts payable and accrued expenses		232
Property and equipment purchases included in accounts payable and accrued expenses	$ 66	$ 88