Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (51,501)	$ (77,655)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,199	12,887
Depreciation and amortization expense	5,803	5,869
Non-cash operating lease cost	1,683
Accretion of discount on investments	(87)	(199)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,772	(1,461)
Accounts receivable	(1,717)
Deferred revenue	(17,709)	(16,328)
Accounts payable	(1,857)	106
Operating lease liabilities	(3,164)
Accrued expenses and other current and long-term liabilities	(2,895)	1,261
Net cash (used in) operating activities	(62,473)	(75,520)
Cash flows from investing activities:
Purchases of property and equipment	(772)	(2,133)
Purchases of investments	(34,407)	(21,832)
Sales and maturities of investments	8	118,887
Net cash (used in) provided by investing activities	(35,171)	94,922
Cash flows from financing activities:
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	60,527
Proceeds from exercise of stock options	145	136
Proceeds from issuance of common stock and restricted common stock	153	26
Payments of employee tax obligations related to vesting of restricted stock units		(196)
Issuance of common stock under ESPP plan	296	257
Net cash provided by (used in) financing activities	61,121	223
Net increase (decrease) in cash and cash equivalents	(36,523)	19,625
Cash, cash equivalents and restricted cash at beginning of period	85,933	37,601
Cash, cash equivalents and restricted cash at end of period	49,410	57,226
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	54	$ 242
Reduction of right-of-use assets and lease liability from operating lease modifications or reassessments	$ 154