Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (70,875)	$ (51,501)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,122	6,199
Depreciation and amortization expense	5,069	5,803
Non-cash operating lease cost	1,693	1,683
Accretion of discount on investments	(105)	(87)
Amortization of debt issuance costs	329
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,054)	2,772
Accounts receivable	(1,608)	(1,717)
Deferred revenue	(11,917)	(17,709)
Accounts payable	1,032	(1,857)
Operating lease liabilities	(3,288)	(3,164)
Accrued expenses and other current and long-term liabilities	919	(2,895)
Net cash used in operating activities	(75,683)	(62,473)
Cash flows from investing activities:
Purchases of property and equipment	(365)	(772)
Purchases of investments	(125,995)	(34,407)
Sales and maturities of investments	42,804	8
Net cash used in investing activities	(83,556)	(35,171)
Cash flows from financing activities:
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	244,020	60,527
Proceeds from Securities Purchase Agreement, net of issuance costs - related party	20,000
Proceeds from exercise of stock options	12,065	145
Proceeds from issuance of common stock and restricted common stock	120	153
Proceeds from at the market equity offering, net of commissions	24,772
Issuance of common stock under ESPP plan	462	296
Net cash provided by financing activities	301,439	61,121
Net increase (decrease) in cash and cash equivalents	142,200	(36,523)
Cash, cash equivalents and restricted cash at beginning of period	65,126	85,933
Cash, cash equivalents and restricted cash at end of period	207,326	49,410
Supplemental disclosure of cash flow information:
Cash paid for interest	1,843
Supplemental disclosure of non-cash investing and financing activities:
Issuance costs from public offering included in accounts payable and accrued expenses	272
Securities Purchase Agreement issuance costs included in accounts payable and accrued expenses - related party	100
Proceeds from exercise of stock options in prepaid expenses and other current assets	$ 61
Reduction of right-of-use assets and lease liability from operating lease modifications or reassessments		154
Property and equipment purchases included in accounts payable and accrued expenses		$ 54